That's it, I've beaten Civ6 in Deity, I simply browsed my small loading game screen, took back my first game (the one I said I abandonned because of a cultural civ and Gilgamesh), and saw it was totally feasible. However, just like my win in Civ5 Deity, if I would have competing against other players I would have been completely wiped out : Gilga started to build ton of Giant Death Robots and sent them to Arabia. During all the final phase of science victory, when i was building the exoplanet project and such, he was like : "best buddy for life isn't it ?" Ok bro, best buddies ! The funniest in it was that he seemed sincere because he didn't spied on me whereas it would have been MASSIVELY profitable to him. The cultural civ never passed its "nearly cultural victory", maybe because i attacked it and had to build armies ? However it had a ton a works of art, but my spies couldn't stole them (happened as if he had none in any city, but in the diplo screen it showed dozens and dozens of them). Is this a bug or some civ features ?
Anyway. I still think Civ6 Deity, not really talking about AI smartness, because in every game it is all relative, and compensated with bonuses, is harder to beat than Civ5, not because it's smartest (I would say it's weaker, indeed), but because of its bonuses and especially early ones, 3 cities + a bunch of units.