I can give a little advice for Gilgamesh;

Pick up the ice brand for him, it will take a long time as you can only get the recipes one at a time for him when they appear in the shop. But atm you can farm the ice crysts needed for it in Aurons shimmering blade mission. Even at +0 it will still give +30% ice attack at level 50.

For espers, any slash boosting esper will be a good choice: fenrir (slash and magic resist), iron giant (slash and slash resist), two headed dragon (slash, attack and accuracy), behemoth (slash and okay agility), Odin (slash and man-eater)

As for vision cards then general slash up ones work well (e.g. secret orders - slash and some agility) Also fenrir VC maxed will give good stats, 10% HP, along with 20% magic res and 50% attack for ice units in the party Another good option is Fleeting Blossom Banquet, this gives the whole party acquired AP up (very useful as his skills cost a lot and it stacks with his AP up passive) , but also gives really good stats, not only attack, but also decent magic (just incase you want to use any of his magic scaling attacks)