The first big update for the Puzzle Creator has been released today fixing a whole host of issues including optimizations and rendering improvements as well as fixing rendering errors which plagued some users. Full changelog:
I believe the "goo" that allows portals to be made on it is actually considered to be paint. Though I could be drastically wrong as it wouldn't make much sense for buttons if that was the case.
Goo means water. Before if you tried to put a paint sprayer vent over a patch of water it would give you an error that the paint vent was in an illegal position.