You can use a barrier coat between the epoxy and gelcoat. I think duratec has the primer. From their website:
Will Duratec adhere to epoxy resin laminates?
Duratec Products will adhere to most epoxy resin laminates if the epoxy surface is properly prepared. First, water-wash with a Scotch-Brite or equivalent abrasive pad to remove any amine blush, then sand with 80 grit sandpaper. Wipe down, then apply Duratec. Always test the adhesion between systems that use different chemistries, such as epoxy and Duratec.
http://www.duratec1.com/FAQ.html
http://www.merrittsupply.com/products/28261-duratec-surfacing-primers.aspx
Will Duratec adhere to epoxy resin laminates?
Duratec Products will adhere to most epoxy resin laminates if the epoxy surface is properly prepared. First, water-wash with a Scotch-Brite or equivalent abrasive pad to remove any amine blush, then sand with 80 grit sandpaper. Wipe down, then apply Duratec. Always test the adhesion between systems that use different chemistries, such as epoxy and Duratec.
http://www.duratec1.com/FAQ.html
http://www.merrittsupply.com/products/28261-duratec-surfacing-primers.aspx