how did they meet? well i decided to make smth that i have never seen someone else do before to my knowledge so here are some lore
countless genocides were made by the human and gaster had always tried to stop them in any way possible he tried many attempts these other attempts being making papyrus battle with sans with his influence(undertale help from the void) or just influence sans with knowledge(undertale last breath) and more and all failed so he decided to change things he made sans murder monsterkind to stop the human(dusttale last genocide) but failed he influence another world(underswap) and did the same but a little different(dusttrust) and even dangerous deals(undertale something new) and even doing experiments on sans driving him insane(undertale insanity) all of these failed until he decided to put them all together four murderous forces surely they would win right? well there's so much to show you
this is all the lore i wrote up so far lemme know what you think in the comments