<Snipped quote by Cu Chulainn>
Forgive me, but the GM mentioned that he's looking for characters up to Gilgamesh's power level (not that I'm using him for power explicitly), which naturally includes Gilgamesh himself. You banning overpowered characters in a roleplay about overpowered characters makes the premise of this story very confusing to say the least.
Otherwise, say I pick Arjuna and still get banned because he too is too powerful (being just a sliver weaker than Gil) and so and so forth until we are down to the level of normal Servants again.
This smells kinda like false advertisement for me.
I feel like this argument is made in bad taste when talking about Gilgamesh specifically when we are talking about Servants. Most Servants are limited to a certain theme that keeps them balanced with the rest in some way. This compatibility keeps even the strongest Servants around the same balance as weaker Servants since they are limited around a certain theme. Many Servant fights are scored by compatibility, in this case.
Gilgamesh on the other hand is one of those outliers that can easily be abused due to his signature Noble Phantasm breaking that balance by having a Noble Phantasm that essentially breaks the rule of compatibility. Even though Gate of Babylon is stated to not have specific Noble Phantasms within its vault, there's still the fact that it has an answer to essentially everything else, and smarter players can easily play around his restrictions and win a Grail War with ease. In other words, compatibility is essentially thrown right out the door.
This is why Gilgamesh works well in normal written with his personality in mind, but terrible in works where writers aren't cooperating with each other, like in the case of a roleplay. There's also the fact that the main GM mentioned that the power has been scaled down slightly to "Cu-to-Artoria/tier"