They tilt their head. That's OK? "Don't think that was me. I never met a Gaster in my timeline. ...My two, I guess," they add, trying to study his expression. They did RESET once, after Asgore died--they're a little more interested in if he knows what they're hinting at.

The creator-Gaster of their younger family--the brother-Gaster of one of the Sans-es they thought of as family too had let himself be experimented on--both of them knew about 'the Anomaly'. But that doesn't mean all of them do.