I thought about this, actually! I mean logically, you'd think Asgore would put the pieces together. I even wrote out several scripts following the idea, but in the end I didn't go for it. It'd be a long tangent at the end of the comic for one, but mostly because it just... didn't go anywhere. It dead-ends in an unsatisfying way where nothing really changes or gets resolved, so I just set it aside.
You're on the right track about him deciding not to push it! But basically I figured, if Asgore came and asked...
Gaster: he wouldn't tell him, in part because he's afraid of how Asgore would react and doesn't want to lose him, but also because he doesn't want to talk about something that personal to the brothers without their permission. Especially after Sans told him that he didn't want him to talk about what happened to them with anyone else! He'd probably talk around the issue, if he didn't outright lie about not knowing, and point Asgore in the brothers' direction so they could decide for themselves if they wanted him to know.
Sans: Already said he doesn't want anyone to know about what happened. 1-S is dead. He'd fend off any questions Asgore asked with silly joke answers and completely stonewall him on it.
Papyrus: Also doesn't like talking about "that stuff" with other people, and didn't like the pitying looks that Asgore gave him about his scars. He's with Sans in wanting to move on after what happened, and he also wants to keep Gaster in good standing with everyone and in a healthy living situation with Asgore. He'd fend off the questions in a way that'd make it clear that he was uncomfortable with the issue (much like how he fended off the kid's back at the beginning) and I feel like Asgore would sense that it's an uncomfortable topic and not press too hard.
So while Asgore might be suspicious and might figure out that the plates and Gaster are connected, none of the skeletons would actually tell him or explain anything about it, and since they've told no one else about what happened and no one remembers it, there's no real other source of information about it anywhere. So Asgore ends up empty-handed, and the reader would likewise feel disappointed that the whole thing ended up going nowhere. It'd just feel like a waste of time, I think, especially so close to the end of the story.
It is something I thought about though! Probably something better left for a fic or something, really...
