Gerson is a boss monster and here's why
As we know boss monsters age when their offspring does and if said offspring is dead they are stuck at the age they were when their offspring died
And we know Gerson is dead in Deltarune
And the surrounding graves are of characters who we know were dead in Undertale as well (which is interesting that more of the amalgamates aren't there which is either due to lack of space or it means they are alive in the Deltarune universe)
(Images from a post by @seased )
And we know that the amalgamates are dead
When monsters die they dust.
And we know Father Alvin is Gersons son
So why is Gerson dead in Deltarune where he has a son but not in Undertale where he doesn't? Because he's a boss monster that's why

















