Autobody paint shops want to charge me an arm and a leg to have parts coated in urethane clear coat. Is using something like GC50 (https://www.easycomposites.co.uk/clearpol-iso-npg-clear-polyester-gelcoat) going to be comparable to urethane clear coats?

hm.....may have wrote this topic too soon. Noticed that the technical questions section of GC50 says that this can't be used on top of epoxy when skinning:

If anyone knows of any alternatives to my predicament, please let me know

Just use a standard acrylic clear coat. If you intend to spray it yourself then just be aware that spraying any chemicals poses poses a health risk and you should wear proper PPE. Spraying acrylic doesn't require anything serious though, just ask your local paint shop what they would recommend.