I am using some 3/4" FR4 epoxy fiberglass pieces to mount some Zipwakes on the slightly curved transom of my boat. Just wondering if I would be able to spray these with white gel before installing? Not sure if these fabricated sheets would have the same characteristics / resistance to gelcoat as an epoxy repair?

Post up some pics for what your making out of that stuff.
I'd call and ask them about the gel

If it's the same FR4 as what they use for fire retardant boards (G10 is just w/o fire retardant), then you very well may have an issue with gel adhesion as the FR4 is an epoxy /glass based material... call the mfg.

Thought I would update some results on some test pieces that I sprayed. First, am I correct in assuming any incompatibility issues show up only as the gelcoat not curing properly? If that is the case then, it appears that the FR4 (black) is a suitable substrate for gelcoat with just standard preparation methods. The gelcoat cured in a timely, uniform manner when sprayed thick or thin and also when sprayed on unprepared, not sanded areas. Materials and application were Evercoat One Step 5670 gel (with wax I assume), thinned with acetone and sprayed with a Preval sprayer.

Curing wouldn't be an incompatibility issue, i got cured gelcoat and resin all over the driveway and shed floor, hell the overspray is still sticking to the truck windows after 4 years, except where the wipers wipe ha !
if it cured like you said than it must have been gel with wax or it wouldn't cure in open air.

Adhesion is your main concern, preparation is key.

Just wanted to bring this to a conclusion. Yes, you can gelcoat FR4 panels beautifully without doing anything out of the ordinary prep wise - at least with the stuff I bought. The bond of the gelcoat was tenacious on the sample pieces I shot. The gelcoat finished out beautifully on the final product. It was interesting that when I sprayed over some marine-tex used to cover the counter sunk attaching screws, the gelcoat turned off white and would not cure making for these perfect little circles when wiped off. I did use the standard protocol to account for the epoxy amine blush. Another little trick that appears to work well is it appears that you can successfully tint 3M 4000UV with standard gelcoat tint making for a nearly invisible calk job. Time will tell on all of this, but everything appears stable and permanent. I really had quite a few hours in this Zipwake project when you consider a making them work with a curved transom, running the control wires hidden under the interceptors and a couple new dash panels so the controller looked factory mounted. Luckily, I had a nice place to work on all of this (work!) and the results were well worth it. While I have never used trim tabs, I cannot imagine them working any better than the Zipwakes. The difference is quite remarkable when turning the system on and off, especially in choppy water.