But why does GF have such an ability that could make you forget something only when you equip it for the first time? I don't know how it exactly works. What would happen if Irvine first-equipped GF like what others did? Would they act like nothing happened and moved on? Bad or good?

The gardens NEVER mentioned any of this either, like they didn't even tell Squall and others about the bad impact of equipping GF.

The gardens are a part of ultimecia's plan, right? GF as well?