It is generally not recommended, but for very small repairs, you can apply gelcoat over fully cured, cleaned, and properly prepared epoxy. For bigger applications, we recommend that you remove any amine blush, clean, sand the epoxy smooth, apply a form of tie-coat primer (generally an epoxy primer such as Interlux Interprotect, TotalBoat TotalProtect, or TotalBoat 2-Part Epoxy Primer), then sand to give the gelcoat a surface to bond to. The epoxy tie-coat primer cures to an inert form, and accepts the mechanical bond of the gelcoat far better than a structural adhesive.
Epoxy fairing materials such as TotalBoat TotalFair will prevent gelcoat from curing, and gelcoat should never be used directly over TotalFair. Even sanding heavily prior to applying the gelcoat will not correct this problem.