I just can't see how Gaster WONT have a major apperance in the future.
It'd be hack writing if the one of the game's central themes is about showing compassion for seemingly unimportant individuals because they matter to you, to give those that have been "discarded" new purpose, only to go "EXCEPT FOR THAT GUY, YOU MIGHT REALLY CARE ABOUT HIM BUT HE ACTUALLY DOESN'T MATTER!".
>but casual players will just be confused if they see him
Like everyone who doesn't know of Gaster's existence is probably thinking "who the fuck is this narrator person and why is Deltarune so important to him?". If that guy is Gaster, the game might as well have to explain him at some point. Fucking Undertale exposition dumps a lot of critical info at you in the true lab and it's right before the final boss. I'm sure casual Deltarune players can handle the sudden introduction of a guy who was never name-dropped a single time before, with the realization that he was with them the entire time only now they know what his deal is.