Gith became resistant to the mind control over the millenia. This is why in 5e the race has psychic resistance. The duergar were also slaves of the mind flayers and this is why they have psionic fortitude. There's some other races that have been altered due to being enslaved (derro, kuo-toa, quaggoth), usually resulting in some form of psionics and madness.

Part of what makes mind flayers interesting because their society touched and left scars on a lot of other races. Gith are just the loudest about it in part because they live in the Astral Sea where time doesn't age them so, outside of dying during raids, many of the gith who rebelled can easily still be around leading to the rebellion being fresh in the gith's minds (as opposed to the duergar for example who live on the material plane in normal time and have had empires rise and fall and numerous generations since the enslavement).

1. Escape temporarily with a group
2. Find scrolls of modify memory
3. Remove memories from all but one escapee
4. Rest of the group returns home with no memory of escape or the person left behind
5. Escapee on the outside learns the spell modify memory and/or finds more scrolls
6. Returns to orchestrate resistance
7. The resistance operates in cells which have no memory of being part of the resistance
8. "Handlers" are rotated frequently and are responsible for providing memories and stockpiled weapons
9. Cell members are given memories, perform missions, and then return home with no memory
10. Cells that are compromised are abandoned immediately and their members are never activated again.
11. Handlers are personally recruited and serve for short periods of time before being wiped by the next handler.

Seems like a pain, but possible. It would make a good story.