The AI is certainly not playable. It will collapse, without fail. What difficulty you play on determines when the collapse will happen- GA might last until 2300, the lower difficulties its within 30 years of the start.
At this point I did 6 runs with GA , there were on 36 AI evry game , 8 revolts over ALL games . The AI even when is a minor faction , had at max 1 resources in red .
GA difficulty doesnt crash on itself , it just slow down its scaling in comparison to a player . Most of it happens with the coming of battleships and cittadels tech. Because most AI in game will not have the income of alloys to produce battleship / the alloys in bank to / buy alloys from the market to create . Only major AI empires will build battleships consistently . This mean that in a game , usualy only 3-4 AI will have aauto fleet power capable of being a treath to a player . But theyr use of the fleets wil make it way easyer .
This mean that the AI will be able to be a treath only till 2300/2400 when you can massproduce battleships with the right shipset , and most AI will fall behind -> you can mass conquer , the other major AI could do it too , but theyr fleet use will make them slow and clunky at doing it . Even when it has 1M fleet power , it will take years to comquer another empire that has 120K .
The growing size of the empire is another factor , since the AI mamage it badly aboud shipyards , fleet positioning , not building gateway .
The GA difficulty work , it just is inferior to experienced player after 2300 .
Edit : and , as I said before , theyr economy building for some reason is BETER than then normal difficulty , I can take controll of most of theyr empires , so removing the economy bonuses and still be completly in green . For some reason , giving the AI cheats make the AI build beter economy .