How high gear lvl do I have to get my phoenix squad for thrawn?

For most of the significantly difficult events- Thrawn, BB-8, R2, the heroes' journeys- G8 is considered the minimum. With great mods and the blessings of the random number god, it's possible.

Minimums are always bad advice. Always aim higher. That said, for Phoenix squad, punching through the G8 wall is difficult. It is easier to get some of them to G10 than all of them to G9. Strong contenders for that are your attacker (probably Ezra) and Kanan.