Make a portal to the one of the following locations for interesting effects. 

Planet - Venus  - Its essentially a planetary scape version of some spells.
Planet - HD 189733b (aka rains of terror). It rains glass....sideways.....at mach 7. imagine what that would do to anything in front of it.
Planet - Neptune or Uranus - it rains diamonds....so  yay spell components

If you open a Gate to someplace awful (Abyss), and force someone through, can they use their turn to just walk back thru? 1. Cast Gate in ambush location, 2. BBEG shows up, 3. Use my Action to push them thru (Repelling Blast), 4. Does BBEG just use his action to re-enter Gate from other side before I can drop Concentration?

game master 

Jusr use a pseudonym, like all fiends, liches, etc. It's your own fault for giving anyone your real name in a world where names have so much power.

Quote from QueenTiamat97 >>

Can I use this spell to summon Tiamat?

In theory, maybe, but in MToF it notes that she appears to be trapped in Avernus by a deity-level curse. That absolutely has fiat to overrule any printed spell in the game.

I would say Tiamat would be out of Avernus a lot faster than Rise of Tiamat, if one of her Cult members could just Gate her out, or any invading Deity with a cult.

I know, but in theory there's nothing in the spell's description that outright prohibits using it to call a deity onto the Material Plane. DM absolutely has fiat to say they're not interested or the rules of the universe or a divine accord blocks it or something, but it's not explicitly impossible per the spell description.

Exactly! You're telling me that someone who's made a pact with an otherworldly being can't open a gate to another world?? 

Quote from LordCre1970 >>

If you open a Gate to someplace awful (Abyss), and force someone through, can they use their turn to just walk back thru? 1. Cast Gate in ambush location, 2. BBEG shows up, 3. Use my Action to push them thru (Repelling Blast), 4. Does BBEG just use his action to re-enter Gate from other side before I can drop Concentration?

 You can drop concentration whenever you want, so it shouldn't be a problem. Shove 'em through and immediately drop it.

Quote from The_Ace_of_Rogues >>

Quote from QueenTiamat97 >>

Can I use this spell to summon Tiamat?

 

In theory, maybe, but in MToF it notes that she appears to be trapped in Avernus by a deity-level curse. That absolutely has fiat to overrule any printed spell in the game.

The portal is also only 20 feet in diameter. Personally I'd say it's too small for Tiamat to squeeze through though that is ultimately up to the DM.

What if you cast this on the ground to drop somebody in?

So target the ground underneath somebody, making it larger than their space so it's technically unoccupied. Then cast Gate, dropping them in.

Would that work? If so, then I have some ideas regarding Sul Khatesh's combat strategy.

You can't target the ground underneath someone because that is not an "unoccupied space"

Quote from grizzyGR >>

You can't target the ground underneath someone because that is not an "unoccupied space"

 Good point. Nevermind then.

If you’re a warlock and know your patron’s name, this may as well be called Summon Otherworldy Patron. 

trying to find the rules for this, but if one of my players has the cubic gate, does that negate the material component aspect?

Yes, the cube allows any creature to cast Gate as an Action, without needing a 9th level spell slot, or any verbal\somatic\material components.   
They do need to maintain Concentration though... (Magic items don't relieve Concentration unless their description says they do, or unless they describe their effects as being 'as if' a particular spell were cast, etc.)

GM means Game Master.

Its a term used because dungeon masters has a sexual connotations. And back in the old days. We used GM because not all games were in dungeons.

So I'm seeing a certain kind of tactic take shape with this on a warlock.

If the warlock has Repelling Blast, Metamagic Adept with Quickened Spell, demiplane from the 15th level Mystic Arcanum, and gate from the 17th level Mystic Arcanum (thanks to Tasha's Cauldron of Everything optional rules), they can possibly do the following (assuming the DM doesn't rule against it for some reason):

1.) Create a demiplane intended as a prison with the demiplane spell before anything happens.

2.) In combat, cast gate to create a portal to your demiplane just behind an enemy, and make it a bonus action with Quickened Spell (gate is once per long rest anyway, so reserving the Metamagic Adept sorcery points for it is fine imo).

3.) Use eldritch blast with Repelling Blast (which you have up to four attempts for at that point) to push the enemy through the portal into the demiplane.

4.) Drop concentration on gate to close the portal before the enemy can run back through it, effectively sealing them in the demiplane.

It says an unoccupied space the portal opening its self is an unoccupied space. Open a 2nd one in the 1st one and see what happens. I think it will act like a bag of holding and portable hole just on a massive scale

Quote from Gavros963 >>

When will it be updated so that I can put this on my warlock build?

 good news, Tasha introduced it to warlocks.