So the molded composite wing tips and empennage tips come with a white gel coat on the outside. Will paint properly adhere to this stuff, or should the gel coat be sanded off before painting?
Sanded yes, off no.
I hope you are not getting ready to pull the trigger. The reason I say that, no offense, is because 90% of the paint job result will be the prep work and that question seems like you are not a paint guy, again no offense intended. If you sanded all the gel coat off you would end up with a big mess, fibers/fuzz, the part would be toast. On the translucent parts you have to fill and sand to get rid of pinholes. Both take work to get the results you want.