First time pre-shader question

Personally I would just mix a very dark flat brown or something of the nature. I would be afraid of some of the gloss still showing through, but if you are sure that the paint will be suitably misted over the gloss, it should be fine. Why not just try it on a piece of scrap?

Ive used the gloss paint for pre-shading with no problems. Even if the glossiness did show through, I think it would be taken care of in the final flat coat anyway.

With those colors (I'm assuming this is your Wildcat?), you may want to consider using a dark gray or dark brown (or gray-brown) instead.