Quote from TheAlmightyN >>

Yep... assuming Tiamat is currently on a different plane than you are.

Just one problem:

Deities and other planar rulers can prevent portals created by this spell from opening in their presence or anywhere within their domains. BR, pg, 244, or right above on this screen.

I'm pretty sure Tiamat is a planar ruler, which means she can stop portals from opening up near her.

So my DM homebrewed a magic item called a Gateway Disk. Essentially, it grants me a Gate once per day or Plane Shift to Planes for which I have an ephemeral key. (Very homebrewed.) On a scale of one to ten, how broken is this item?? (I need to clarify some points with him; I may be exaggerating something.

He says its a legendary wondrous item. 

What happens if something hits the edge of the portal? Does it bypass the portal or would it get chopped in half or something like that?

This spell is SO OP.

How to permanently seal a tarrasque or Tiamat:

1.) Demiplane spell, make a new demiplane.

2.) Mordenkainen's Private Sanctum for 30 days in a row in the demiplane.

3.) Cast Gate, say whatever you want to trap.

4.) Plane Shift away.

EDIT: May not work with Tiamat (see SharkLaserBoy's comment), but depends on DM.

Quote from noteveryoneisdead >>

That moment when the GM gave you a magic scroll with this spell on it for a plot point but you immediately use it and say "Tharizdun" instead of the demon you were meant to kidnap/kill and in doing so you essentially end the campaign due to the Chained Gods ability and drive to destroy everything

 two things:

1. is Tharizdun the demon's True Name? 

2. Deities and other planar rulers can prevent portals created by this spell from opening in their presence or anywhere within their domains.

Yes, but it only lets you take the creature to the plane you are on, so you would have to survive in the fire plane long enough to cast the spell, then find a way back to your own plane without letting that person out. I think its main use would be for summoning demons or stuff like that. 

Might be a silly question but is the concentration necessary for the gate itself only or both the gate and the presence of the summoned being?

Also if the presence of the summoned being does not depend on concentration, should their XP be included for the players reward ? 

The concentration is only necessary for keeping the Gate open, the creature being pulled through the gate to you doesn't require concentration once they are through.

For the second half of the question I'm not quite sure what you are getting at, but I'll try answer itfrom a few different angles.

If you mean if the creature pulled through gets a cut of the XP from a fight you pulled them into against their will, then probably yes but that's down to your DM.

If you mean do you get XP from killing that creature that would be down to the DM

Ty for the answer. I'm the DM though so.... haha I guess I'll figure something out ;) 

I don't think this should be 9th level when plane shift and teleportation are 2 spell levels below this and only lasts for a minute.

Make a gate to a star on a diffrent plane they all have wacky lore and if they rule it as a normal star you would incinerate everything and die slightly after your enemy. If the DM ruled the death ray as going in a straight line rather than fanning out bbeg dies instantly unless they have unreasonable plot armor. I also used a similar method instead with a bottomless sea of lava. Killed the dragon by drowning.

Why is this 5,000 gp to use?!

Quote from Iron_Sir_Levi >>

Why is this 5,000 gp to use?!

I feel like it's reasonable for a spell that opens a portal to anywhere in the multiverse to be a little expensive

To be fair it's not every time. It clearly states that the diamond is needed not that it is consumed. 

Quote from AlwaysChaoticNeutral97 >>

Can I use this spell to summon Tiamat?

 Assuming Tiamat is her real name and not her pseudonym

Is an intelligent item a creature in your world? It sais "speak the name of a specific creature". So... If the item is viewed as a creature, you're good to go!