This spell is poorly written. As written, a creature under its effects can be

• Grappled
• Restrained
• Shoved
• Affected by spells that affect humanoids like hold person and charm person
• and physical spells like spike growth do full damage to it.

Even though you are a misty cloud.

Now a DM could certainly rule these things don't work. But they should have just addressed them in the spell.

How I'm using it is a use rats as fodder and ammo. Spellwrought tattoo on a trained rat that I can communicate with. (Ie shatter) ..then gaseous form them into an enemies lungs...and boom!...cancle concentration and rat uses shatter inside ur enemy. 

You can fit though small holes and cracks. RAW they can literally just use their movement to leave a grapple. As any grapple is going to have space the gaseous form can leave through. 

You are no longer a creature. You are “transformed”, creatures can be grappled as per the grapples description, but the spell says you are no longer a creature. That was a ruling my DM made when someone attempted to heal me. That should be enough to make it make sense, as well as passing through holes, cracks . But that one alone isn’t enough since you can be melee attacked as normal, and be dmged, though you have resistance

Quote from Rationale01 >>

You are no longer a creature. You are “transformed”, creatures can be grappled as per the grapples description, but the spell says you are no longer a creature. That was a ruling my DM made when someone attempted to heal me. That should be enough to make it make sense, as well as passing through holes, cracks . But that one alone isn’t enough since you can be melee attacked as normal, and be dmged, though you have resistance

 Right... then why does it say the spell ends when the creature drops to 0 hit points? The target is still a creature and it doesn't directly say that they are no longer a creature when you're in your mist form. Your DM misruled and I can't really think of a reason that they would have aside from being antagonistic. 

Light a match? That's my.guess

Quote from DVision >>

This feels like a 1st level spell to me. I can't justify it to my self as 3rd level when fly is a thing. Can someone give me a different take on how to consider using it?

Fly doesn't get you through door cracks, pipes, jail bars.  This is more or less a problem solving spell for getting in/out of somewhere that is locked down pretty tight.  Its hard to imagine a place in D&D settings that doesn't have some imperfections in the stone, wood, or structure that would allow gas to seep in or out of (save magically warded).

While it can be doubled up as a movement spell that isn't its primary function and would really only be used in pinch as opposed to fly which is both faster and less restrictive on what you can do while flying. 

It is definitely far too powerful for a first level spell, as there are too many extra effects of advantages and resistances to drop this to a level one spell.  Otherwise you'd have a level 1 spell that gives resistance to all nonmagical damage, and advantage on 3 different saving throws (plus flight & corporeal form). 

It can be a decent defensive spell as long as you're okay with the level 3 cost and the loss of actions but it would be cheesed (abused) if it were a level 1 spell.  Though I think there could be an argument for 2nd level.

There are a couple of ways to use this spell in combat.

One is the dragon breath strategy:
1. Be a dragon born or have access to the potion of dragon breath.
2. If you don't want to be a Dragonborn, have  access to a potion of gaseous form and cast the spell dragon breath "you just can't have two concentration spells"

In the spell Gaseous form and its counterpart potion of gaseous form, "The target can't attack or cast spells" 
 have an effect on you like the racial Dragonborn breath or a potion of dragon fire.

Dragon Breath does not count as an attack, because there is no attack roll involved, but a saving throw instead.

This is confirmed in the language in the players' handbook, and Xanathars spell Dragon Breath. With further proof by Jeremy Crawford ruling on familiars with similar language to gaseous form. 

Jeremy Crawford

"Dragon's breath is cast on a willing creature. Your familiar is a creature that can be willing. Find familiar prevents a familiar from attacking, but dragon's breath involves no attack."

 >>>>>>>>>

To further expand on this spell with effects like:
Scourge Aasimar - Radiant Consumption
Druid-Circle of Spores Level 2: Halo of Spores/Symbiotic Entity
Spirit Guardians

Just keep in mind the effects that need concentration (You can't concentrate on two spells), and even then, there are ways around that like potions and spell storing Glyph of wardings

^^^^^^^^^^^
So all that being said why would you do this.
*advantage on Strength, Dexterity, and Constitution saving throws.
*resistance to nonmagical damage (This does not say resistance to damage from nonmagical weapons. So this means resistance to ALL none spell and magic like affects. Lava, nonmagical poisons, Breath from dragons, ect )

other than the limited movement speed which can be avoided with long strider or adding rogue BA dashes this can be a top-tier buff spell.

If I cast gaseous form on myself and then get inhaled by a creature, does that count as getting "swallowed," like in the case of a giant toad or a tarrasque?

On a related note: if I get inhaled by a creature that is the same size as me or smaller, and then drop or lose concentration, do I take bludgeoning damage when I resolidify? Does the creature that inhaled me? Do we both?

this was posted a while ago but if anyone sees this, the party wizard could cast this on the heavy armour user to help with sneaking around. and the person in gaseous form could also move into a little glass vial that could then be thrown at a bbeg, at which point the glass will smash. wizard drops concentration and then bam the fight/bard/paladin appears right in its face ready to bring the hurt. we use me as a paladinade complete with mage slayer 

Theoretically you could use this to kill your enemy as long as they are or the Huge size.

An elephant has a 310L capacity for oxygen in each breath. That's 155L per lung which can be converted into 5.4 ish Cubic feet. 

Big enough for a Medium size creature to fit without compressing as your gas form.

The spell already states you can fit in your enemies space as well as squeeze through spaces as small as a key hole or crack.

Get it, cancel concentration, rip and tear, win. Just make sure to have another spell slot so you can get out after.

Arguments against?

Slightly odd question. Can someone who has been turned into a gas take a short rest while in gaseous form? Or is that a "You're the DM, you get to make that call" kind of deal?

I see your point about this spell, that it doesn't fall in-line with the idea of being a cloud. Maybe they should re-name it to amorphous floating form. Or maybe the DM can rule that this spell allows you to have those features that you mentioned?

Personally, I think this makes a good spell to use to get into places that might otherwise be difficult to get to or even impossible. Like a trapped room, you could float through a tiny vent. It might also be useful for scouting as this form might not draw the attention of those untrained in Arcana. Lastly, I'd Consider using it to take a short rest, though some DMs may rule that you can't rest while concentrating on a spell. But you can use the form to sneak away somewhere secluded like a vent shift you wouldn't be able together to otherwise. Though, it's not the most elegant use of the spell. I suppose escaping a jail cell might be the best use of this spell. 

You can drop concentration :3

what would happen if this was cast on someone under water?

If cast upon a willing creature carrying a bag of holding, can you safely put the willing creature and everything it was carrying into another bag of holding without causing a rift to the astral plane to open for one hour?

bottles

I'm ngl it's kind of hilarious that you can grapple a literal fart cloud