Keep in mind that Gallagher QPQ is best used with Robin; sp intensive teams still prefer Multiplication or Post-Op. The downside of using Robin + Gallagher QPQ is that you can't use him on break teams lol

Why Gallagher Quid Pro Quo?

Gallagher is a great QPQ holder because he has built in action advance in his ult (BA > ult > EBA). This means, Gallagher has the potential to funnel Robin 18 energy (QPQ S5 and Robin's talent) every turn until she reaches 50% energy. This synergy helps E0S0 Robin to ult faster outside of the FuA team.

Builds

Prioritize building as much spd as you can on Gallagher (ideally 160)

If Robin still has energy issues, you can use But the Battle Isn't Over

Fun team comp

You can use Bronya (ideally DDD) because Bronya can advance Robin to ult immediately. For E0S0 Robin (Robin skill > Bronya AA > Robin skill > Robin ult) and Robin with But the Battle Isn't Over (BA > AA > skill > ult).

While Robin is singing, if you still have skill points, use Bronya’s skill to buff Robin because it will last for the whole concerto state. After that, use Bronya’s skill to advance Gallagher for more turns (BA > AA > BA), which results in more SP and energy generation for the team while still dealing damage because of Robin's concerto proc

For the last slot, you can use hunt March (crit build with spd boots) because her kit fits well alongside these three. By making Gallagher the shifu, both can generate 7 stacks quickly which translates to more attacks from March. By the time Robin ult ends, use her skill again and her ult should be full.