I’m no expert on gel coat,but did repair a fiberglass fender on my own boat once.
I done research as how to go about it and did it. Basically a wax is added on the final. Coat of and if you go back over it in an attempt to empty the gun then you creat this problem of trapping the wax that has already migrated to the surface. The wax is what causes the gel coat to harden. And yes I made that mistake and it always looked great right after washing and it only lasted a very short time and it began to look milky. Bugged and buffed to no avail. I think the only solution is a do over. If it bugs you,go seek a expert boat repair.
Not sure of your circumstances on a repair or not,just a thought. Good luck.
Ed