Gil for Short to Medium long fights.
He needs little if any support, but works well with Merlin, HC Anderson or just any buster support.
For farming, he works pretty well with Archer!Isthar.
QSG for Long Fights and Challenge Quests.
To max his potential he needs some star-generator, I find that an Arts/Crit-team work good with him.
Ruler!Jeanne can generate stars and her 3 Arts cards allow for Arts-Chains = NP is quickly ready. And since both can protect the team with their NP, even the hardest hitting enemy with be stymied.
Caster!Tamamo has skill-CD-reduction, more support and 3 arts cards.
HC Anderson is good general support and great at Crit-support.
Saber!Lancelot brings more Star-Gen and an offensive NP to the mix.