If Gilgamesh actually fought seriously, using Ea and Enkidu front and center with support from the Gate of Babylon... yikes. Alright. I'll say it. Potentially? He IS the most powerful Heroic Spirit.

Lancer!Simon, Berserker!Ryouma.

If a certain amount of his pride is broken and if he thinks you deserve him to be at full force when fighting he might do it.