I got 3 free months of PC GP and did play there some old MS games I didn't play back in the time. Soon I'll get a $1 month to play Halo Infinite, Forza 5 and Windjammers 2. Didn't buy the games I played or plan to play on GP, but if I like Windjammers 2 I'll buy it on PS (where I mainly play) or Switch (due to being portable).
I never have been a fan of MS, Zenimax or ABK, from all the stuff they bought the studio that interests me the most is Ninja Theory. I'll get GP a month or two to play Hellblade 2 when released. I have other tastes that matches more the Sony and Capcom stuff plus some indies, and I prefer to buy these ones and if possible in retail (to collect them and maybe to sell them in the future).
I don't have a lot of time to play, so I have a huge backlog and I buy the few games I know I'll (or want to) play even if some of them end will be moved to the backlog before completing them or even before starting to play them.
For me GP (same as PS Plus or Now) is good to play there (and not buy) games that didn't interested me enough to buy them, but that appeal enough to me to play them or at least to test them, even if they are old games. And after that, to don't buy them.