Gat, R2, and Han are all important characters.
I guess I could probably do r2 first, then GAT, then Han, but there's no bad way to do it.
I would base my decision on
1. Prefer, all other things being equal, to give the gear to someone who will gain a gear tier from that one piece
2. Prefer your current high-priority squad.
R2 fills both those criteria. GAT fills the first. Han fills the 2nd.
But really, there's no bad way to do this, and you have to think about what your current high-priority team is lacking. If you are finding yourself very short of offense, then giving it to Han first is reasonable, even though that won't finish a gear tier. I don't have strong advice on this, even if I do present those 2 general guidelines, because those 2 guidelines might be overcome by a lot of other random, small things that have to do with your particular situation and priorities.
You really can't go wrong with any of those 3 toons. This is a case with no wrong answer.