FSN/Zero Gil who has an average Master and is corrupted by modern day society? Archer probably wins since favorable conditions.
CCC Gil who isn't corrupted by modern day society? He wins with low-mid difficulty.
Pure Heroic Spirit Gilgamesh shits on Archer though. Because the rules of the Grail War nerf Gil and his arrogance is kinda affected depending on what era he is summoned in. Ex. Modern day era would corrupt him and he would be too arrogant to calculate plans or anything so that arrogance can be used against him. If he isn't corrupted and is a pure HS unrestricted by Grail War rules, he just smashes Archer.