Haha, servant physiology go brrrr
Servants have passive 4-D Invulnerability, so Shinra can't damage even the weakest servant. Any hax Shinra has to attack Gil also doesn't work and just get hit by 4-D layered Power Null when he tries to hax Gil.
Meanwhile, none of Gilgamesh's regular attacks are going to so much as damage Shinra. However, once he brings out conceptual weapons and Enkidu, Shinra's done for. Even assuming none of Gil's weapons work against Shinra, Enuma Elish will just erase Shinra.
Regardless, Shinrabanshoman can't do anything to Gil, so this is a mismatch.