It's a bit of a trick. When a polyester/glass boat gets laminated they spray gelcoat into the female mold and after partial set continue with the layers of glass and plastic. Can be said that the gelcoat in this case has a CHEMICAL bond with the laminate.
In the case of most repair, restoration, or new added work, gelcoat is the last layer. Most will distinguish this type of layering as MECHANICAL. And the gelcoat is going on as a coating. That's why a DIY looks for the most versatile product that is made for the job. I have heard that cured polyester can be 'softened' with styrene or acetone to help get a 'chemical' bond with a new polyester layer. That would include polyester gelcoat, I guess. Who wants to get into that!!??
We can think of ANY layer or coating as only achieving 'mechanical' bond - like a primer coat or finish paint. These days the most common and probably the best primer/sealers are epoxy based: single or two part, solvent or waterborne. A modern gelcoat product no doubt takes that into account. Paints and coatings can be almost any formulation these days including polyester. The epoxy undercoat usually sticks better and seals and neutralizes surfaces better.
Used on the exterior: gelcoat will protect UV sensitive epoxy. Gelcoat won't stick as good as epoxy, so prep is important.
These days every coating is modified one way or another: powder coating is done with modified polyester that produces good adhesion as a long lasting baked-on coating - usually on metal.
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