watched the main season of SAO

liked the 2nd season but it’s Gun-Gale

That’s good enough IMO. As long as you can recognise some of the returning characters, and not be too lost, you’re good to go!

Also, I should have elaborated in the original comment:

super minor SAO: Alice meta spoiler

The first part (Alicization) of this big arc is the precursor / setup towards the big war. The war only truly starts in the second part (War of Underworld).