"Can't apply gelcoat over epoxy".
That's an oft heard maxim.
Over time I've come across voices that disagree with this as an absolute statement.
One maker even produces epoxy that is intended to be finished using gelcoat (SystemThree's SB-112).
Here's a write-up from another manufacturer. Very interesting and informative.
The key to success seems to be in properly mixed epoxy (no extra hardener) and avoiding/removing amine blush as well as proper preparation for mechanical bond between the epoxy and the gelcoat.