After all, Gilgamesh might have possessed formidable combat power—enough to rank among the very best—but his personality was thoroughly third-rate.
In a Holy Grail War, if you failed to put yourself in the proper place and serve him as a vassal, he would grow disgusted with you. At best, he would abandon you; at worst, he would execute you.
But if, in a Holy Grail War, you did put yourself in the proper place, served him wholeheartedly, constantly flattered him, and let him act as he pleased, then he would naturally start slacking off and having fun. He might even deliberately weaken himself because he found the Holy Grail War too boring, stepping into dangerous traps or even proactively granting Noble Phantasms and elixirs to enemies he found pleasing, all to make his game harder and give himself more enjoyment—before finally throwing the match in spectacular fashion.
Gilgamesh's mindset when participating in a Holy Grail War was like playing a Hextech ARAM match in League of Legends. He had to start with god-tier luck, while the enemy also had to start with god-tier luck but be just a little worse than him. If the enemy's luck was not good enough, he would give them time to catch up, or even hand them equipment himself. And while all those conditions were met, his teammates also had to be at a massive disadvantage, forcing him to step up alone and turn the tide, so that he could enjoy himself to the fullest.
But where in the world could such a perfect situation exist?
If the other side was equally blessed by luck, that meant they had the strength to threaten you. That meant they could find an opening, win one good team fight, and take you out in one go—or sneak attack