CAUTION, LONG STUFF IN APPROACH
About the hypothetical Gilgamesh vs MCU stuff I mentionned. First, none of the parties would know each other. The Avengers had no time to prepare and didn't know who Gilgamesh was, what his powers are, how strong he is, what is his fighting style, etc, so they would be confident. Same with Gil, who had never heard about the Avengers and co and was casual. Of course, the whole "Servants can't be harmed by non-magical weapons" stuff would be casted aside for obvious reasons. So here's what would happen.
First of all, all the low-tier characters (so all humans, pretty much all the Guardians, all the Wakanda soldiers, heroes like Black Panther, Winter Soldier, etc etc) would get obliterated with pretty much a first large salve of Gate of Babylon. And no, the vibranium wouldn't be of any use, first because the weapons of GoB are all magical in nature, way too strong for even Servants like Saber or Heracles and many of his weapons can easily bypass durability. So no more Wakanda, no more sorcerers (Gate of Babylon can shatter Medea's barriers like glass despite them being as though as Hercales'skin and Medea is leagues above even Post-Endgame Strange), no more low-levels. Some could survive by luck but that's the best option. That would leave the heavy-lifters (Carol, Strange, Thor, Wanda and Hulk) and perhaps Iron Man, War Machine and Quicksilver for fun and they're bloodlusted (and that won't do them any good).
First of all, IM and WM wouldn't stay alive for long since Gil can open portals from all directions and thousands at the same time with only a thought (there's a scene in UBW where Gil is observing the progress of his plans, a magical bird created by Rin goes to spy on him and is in his back, Gilgamesh simply opens a portal next to the bird and destroys before the bird can even begin to react and all without turning his back, saying something or even showing he reacted). And that's not getting into the fact that some of his weapons are capable of Homing Attack.
Next is Quicksilver who is uber fast. That won't be useful because even regular Servants can move at the speed he moves. Gilgamesh will easily react to his attacks and open portals in advance to blast him. And let's assume for a second Quicksilver can outpace him. He's still vastly weaker than Gil physically so he cannot land a hand on him. Finally, Gil has Sha Naqba Imuru, his Clairvoyance skill (he keeps it deactivated most of the time out of arrogance but he can activate it with a thought) that is essentially incredibly developped precognition and allows him to watch other world, universes and timelines and know the future so he will know what the opponents will do and stuff like super speed and teleportation will be useless.
Then you have Hulk and Carol. Two powerhouses who can be extremely fast. Let's imagine for a second that Gil cannot simply utterly destroy them with superior weapons like he did with Hercales despite Heracles being normally invulnerable to even the attacks from Artoria and with casual ease. Like said before, many of his weapons can by pass durability, regeneration and invulnerability and directly attack souls. So all he has to do is throwing such weapons and they're toasted. Carol could project space energy or create explosions but Gil can easily summon hundreds of weapons around him to protect himself and be perfectly fine or summon a shield of two to protect himself and reflect her attacks (there's also the fact that, as seen in Strange/fake, he has floating disks that automatically react in micro seconds to any projectiles and attacks to immediately defend him against it and can reflect the shots). Carol is also immensly fast so she could possibly reach him but Gil would simply have to block her for a second, summon a portal, use Homing Attack weapons and distract her in any way to strikes her. Either way, they're dead.
Then you have Thor, who is said to be a Norse God. Two cases here.
- Case 1: Thor is taken as what he is in the movie, an alien who is considered to be a god but isn't a true deity. In that case, he will simply get blasted by GoB with ease during one of his boasts or jumps (if not normally, summoning portals below, above or behind him or trapping him into a bubble of portals will do the deed). Now if he decides to summon a lightning bolt, Gil would have all the time he needs to summon weapons to deflect the thing (assuming he doesn't have his disks, wich would block the bolt with even more ease).
- Case 2 : Thor is taken as a genuine god. In that case, one simple word: Enkidu, magical chains named after Gilgamesh's only friend and can bind even gods and the more divine the target, the tighter the chains. The ONLY instance where a Servant managed to break them was under very particular circumstances for a very particular hero. Thor is the son of Odin, so if he's considered a true god by the chains, he would have a hard time even flinching and then Gil can casually pull out a sword and do another Illya.
That leaves Wanda and Strange. Two powerful sorcerers. Who would do nothing against Gilgamesh because Magic is just useless. Normally, his Resistance to Magic is E with a normal Master and C with a good one so he seems pretty weak. Except he has so many staves, wands, scepters, artifacts and even spells that he's pretty much immune to Magic as a whole. And that's when he's not wearing his armor because his armor alone makes him immune to Magic to a superior degree that even Artoria who is immune against magecraft from the Age of Gods and makes him invulnerable against stuff like pretrification and spatial manipulation (so you can forget BFR him in the Mirror Dimension). Wanda would then simply have her mind hax. And again they would be pretty useless. Because Gilgamesh's ego is so ridiculously gigantic he couldn't be tainted by the six billion curses of All the World's Evil that can blacken Servants with its mud.
In short, a casual Gilgamesh would simply destroy the MCU heroes with casual ease in some minutes at worse. A serious Gilgamesh? Best case scenario, he simply opens the entire Gate, worst case scenario he pulls out Ea (we've seen in UBW that the long opening in Zero was just a show he put against Iskandar to make it more dramatic) and...you don't need me to end that sentence.