Says who? Seriously, this is borderline paranoia some of you have.
Activision, EA, Ubisoft,.... People really thought that Take2 wouldn't try to do the same? Like they saw those people doing more money than before and said to themselves "nah, we are OK, we like Valve and we don't like making more money". Steam is a nuisance for big publishers that why some, like Ubi, became downright hostile on their comments regarding the platform. They feel they don't need steam and in Ubi's case... It seems they were right.
If Epic made the deal, the game wouldn't launch on GoG, hell even the usual Epic deal, the one that pays X number of copies on advance, is damn useless for Take2, why would they care at all, os gonna sold a fuckton of money anyway.
This is a test, to see how sales respond,nothibg more nothing else. If it goes right, you can be sure GTA6 won't be on Steam.