How to defeat Gilgamesh
Once you’ve reacquired the Genji Protorelics from each shrine, you’ll be able to enter the garden. That’s where you’ll have a chance to obtain the Final Fantasy 7 Rebirth Gilgamesh summon. All you have to do now is defeat him in battle.
Gilgamesh is weak against fire, as well as most types of detrimental status effects (i.e. poison and debrave). However, he tends to summon additional weapons, such as the Muramasa, Banana Fan, Rune Axe, and Excalipoor (i.e. fake Excalibur). To pressure him, you need to do perfect blocks or enough damage. Doing so causes him to drop his extra weapons, making his abilities weaker. If ever he arms himself with the Genji Shield, be sure to destroy it as well.
Note that the Excalipoor that Gilgamesh wields has him stumbling around whenever he attacks. However, don’t let this fool you since his sword swipes have a chance to hit hard, likely killing a character outright. You should also watch out for the countdown that he does, as he may unleash his Swordsman Supreme ability to hit your entire team. Proper timing with your Limit Breaks and Synergy Skills (i.e. those that grant immunity frames during the animation) should help you survive longer, too.