Was Gamzee's Dream Self ever seen? I know he was seen in the diagram showing the two teams' dream selves (/?s=6&p=004405), but was he ever there in-story? I have a theory that his Dream Self was hidden away in Prospit's moon's core, similarly to Aradia's, because I don't remember him anywhere.

I’m not sure, but I don’t think so.  I don’t think we saw Equius’s or Eridan’s dreamselves either, but again, I’m not sure.

Your theory could be right but it’s unlikely.  Aradia’s dreamself was in the core of derse because she was dead before she started playing sgrub.

~Mod T