What ships are 'must haves' for the GAS event?

You are good. I beat the ships portion with minimum powers on most with the endurance. If you negotiator it's just overkill

Clone sarge is worth it as he is in the meta line-up currently.

That’s it you’re making a negotiator fleet as well.

Otherwise what you have should be enough.

What you have at 40k is fine IF you boost Anakin up to relic. His is the only ship you need above the minimum.

I also did the mission first pass of the GAS event on my very first try. I hadn't watched any how-to videos or anything. I just wanted t try it for myself. Ahsoka (not yet relic'd) was taken out with a single shot at the beginning of the battle and I started to panic, but the longer the battle went on, the more it was obvious that I could get it done.

And I did.

If I can do it with bad initial RNG on my first try in the first hour the event went live and with no strategy help, you'll do fine.

I won't say it's easy to do it the way i did it. I was biting my nails for a few moments there, but given the opportunity for multiple tries plus all the strategy help now, there's literally zero risk you'll actually fail.