If cast upon one's self, can you dismiss the spell early or are you stuck for an hour? 

 It's a concentration spell, so if the caster wants to cancel it he can just drop his concentration on it, no action or components required.

Can you just fly into an enemy's mouth with this spell, that seems like a pretty solid survival tactic

Shouldn't this spell make you immune to grapples, I mean its basic logic but not listed....

Advantage to Dexterity checks, so no, but a better chance to do so.  You're actually also a better grappler with that advantage to Strength checks.  Con checks make most sense.

This is really stupid, as written this spell provides no advantage on escaping grapples and per the rules it could be implied it's easier to restrain them in gaseous form than not.

Grappling Rules: 

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.

The target of your grapple must be no more than one size larger than you and must be within your reach. Using at least one free hand, you try to seize the target by making a grapple check instead of an attack roll: a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you subject the target to the grappled condition. The condition specifies the things that end it, and you can release the target whenever you like (no action required).

Escaping a Grapple. A grappled creature can use its action to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check contested by your Strength (Athletics) check.

Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.

What happens if someone casts gust of wind against someone in a gaseous form?

Quote from SergeTroy >>

This is really stupid, as written this spell provides no advantage on escaping grapples and per the rules it could be implied it's easier to restrain them in gaseous form than not.

 

Grappling Rules: 

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you're able to make multiple attacks with the Attack action, this attack replaces one of them.

The target of your grapple must be no more than one size larger than you and must be within your reach. Using at least one free hand, you try to seize the target by making a grapple check instead of an attack roll: a Strength (Athletics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). You succeed automatically if the target is incapacitated. If you succeed, you subject the target to the grappled condition. The condition specifies the things that end it, and you can release the target whenever you like (no action required).

Escaping a Grapple. A grappled creature can use its action to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check contested by your Strength (Athletics) check.

Moving a Grappled Creature. When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.

 
"The target can pass through small holes, narrow openings, and even mere cracks" seems to cover it. It's not explicit, but this is one of those areas of the rules you probably don't have to worry about.

Are there narrow openings in any grapplers technique?
Yes.

"I grapple with the gaseous form!"
"It enters your lungs and begins to suffocate you!"

Best not to overdo this sort of thing.

Part of the components list a wisp of smoke, in an rp sense how do you carry around and have in your possession a wisp of smoke?

If you gain this effect as a Potion, can you continue to concentrate on a spell you cast BEFORE you drink the potion?

For example:  Cast Call Lightning.  Use as normal.  Next turn, drink Potion of Gaseous Form.  Does the Concentration last?  Can I still call down more Lightning?

Quote from Greykil >>

Part of the components list a wisp of smoke, in an rp sense how do you carry around and have in your possession a wisp of smoke?

In a bottle, or a stick of incense on the spot (lit during the process of casting the spell).

Quote from Fuzzydragon >>

Shouldn't this spell make you immune to grapples, I mean its basic logic but not listed....

Oh, you’re right.  I missed that on previous readings.

Undoubtedly just an oversight, but still a little strange.

This feels like it should be a first level spell to me cause I only can think of it compared to fly.

Quote from MHM >>

If you gain this effect as a Potion, can you continue to concentrate on a spell you cast BEFORE you drink the potion?

For example:  Cast Call Lightning.  Use as normal.  Next turn, drink Potion of Gaseous Form.  Does the Concentration last?  Can I still call down more Lightning?

As far as I can tell, that should work fine! A potion of gaseous form doesn't require you to concentrate on it, and the gaseous form spell only says you can't cast spells, but using an action granted by one, such as with the call lightning spell, should work!

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?

Casting word is:

Noblus Mistium

You are a wizard. You can carry a wisp of smoke.

Combine it with invisibility from another person, and this is a lot like an Etherealness spell regarding the stuff you can scout with it. Nothing but solid walls can restrain or bind or even hinder you. Can even get you out a forcecage guaranteed!

Also, just how many guards when they see an odd cloud of smoke will think to hit at it until it dies?

Circle of Spores Druid can combo this with their modified wild shape and just exist as a noxious cloud! :D It isn't sn't optimal damage, but fun!

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?

 It's doing a lot more than just giving you a slow fly speed.  You can move through/around many solid objects, as long as they have gaps somewhere - doors, fences, window sills, gates, prison cells, etc.  You're getting resistance to all non-magical damage and advantage on your concentration save if you do take damage.  Many creatures may take no note of a mist, especially at a distance, and even those that do may just be confused by it (such as low Int creatures) and wouldn't necessarily prompt an attack.  Even intelligent creatures wouldn't think to attack it in many cases - running from the guard you round a corner and poof into mist, the guards turn the corner and think it's just some obscuring matter you used to hide your trail, never thinking to attack the mist itself.