See there's a couple of little gimmicks in the GR squad and they involve certain pairs of characters.
When you combine GK and 3P0's skills under Padme, it makes it so that whenever you get crit, you get crit immunity from GK. Then, since you got a new buff, you get protection up which makes you immune to debuffs. The interesting part is that both of these buffs apply before the debuffs attached to the attack whose damage triggered the buffs. This makes your toons have amazingly consistent "perfect" debuff resistance.
Now when you combine GMY with GK, GMY spreading his buffs makes GK taunt. This is an interaction that was verified intentional somewhere a while ago and it is supposed to act like a pretaunt for GR.
What this basically means is the core of the group is Padme, JKA, and GK. The 4 most notable characters to fit in the other 2 spots are: Ahsoka Tano, GMY, 3P0, and Zarriss.
Which 2 to grab really depends on what you're trying to do. Offense or Defense? Which comp are you fighting on O? GMY and AT seem the most situaitonal of those 4 to me. GMY is good for offense, not as good on D IMO. AT is good against teams with amazingly high buff survivability. Interestingly enough this doesn't count Padme mirror matches. She slows down wins in Padme mirror matches thanks to all the undispellable protection up her consistent out of turn attacks generate on the opponent. 3P0 gives this team super versatile survivability as well as his other usual shenanigans and Zarriss has long been known for turning matches into drag out team.
This is why I like Padme, JKA, GK, 3P0, Zarriss. It's a time out team. Everyone's really hard to kill, That's the premium defense team IMO. GMY fits really well on offense, especially against another Padme team since they tank damage so well.
Shaak Ti is another notable 5th, she has some nice abilities that fit in quite well.