Bros, Gaster was shattered "across TIME and space".
He exists beyond time and isn't strictly confined to exist at one point in time.
What if every Gaster dialogue we see is at a different point in time?
It's possible the gonermaker segment is him reaching out to us at the tail-end of his time, where he's about to lose access to communications and is making a last-ditch attempt to connect us into the world, getting shut off/deleted right before we're put into the vessel. We got thrown into Kris as a rush-job and our goal is to get back into the Vessel that's probably in the bunker.
I think it's safe to assume that Kris is an antagonist. It's possible they were the ones that dragged us into their body so they could keep tabs on us and make it certain we don't reach the bunker.
Now, Gaster is still "in" the game but it's an earlier version of him, and we gotta find out what his actual plan is. Unfortunately the Gaster talking to us in the save menu, the ch1 and knight Game Over screens and the end of ch4 are earlier versions of him who just wants to observe things for the lulz and hasn't thought up "the plan" yet.
I think the tweets could be where he ended up after getting booted out of gonermaker, as at that point he had already developed a close connection with the player and that's why we "need eachother".
Maybe he's actually sending us messages backwards in time and when we finally meet him it'll be the very beginning before he's even met us. Then we inspire him to start making the new future, creating a bootstrap paradox.