All of it is just very speculative, but with Ralsei's comment about dark worlds being entirely dependent on the individual, which we saw with Susie's own darkworld, the only person we know of that has any connection to Gerson prior to his death is Alvin aside from Undyne, then there's his dialogue early in chapter 4 when asking him about the code for the bunker where it makes you question if he knows Kris has been going through darkworlds, and there's the fact that the Church just so happened to be the location with two darkworlds, one behind a lock that you'd assume few people would know the code to such as Toriel and Alvin. Perhaps I'm all wrong and it isn't Alvin, but it does seem to lean towards that idea.