Gloss coat question

Try painting the sprue with whatever paint you plan to use, let it dry, then apply your gloss coat. That will give you the same base as your kit will have.

I have never used the stuff, so I can't recommend any thinking ratios. BUT... I do have this suggestion, if you have an old build you no longer care for, select an area of that kit to experiment with. A wing undersurface or whatever. Try straight from the bottle in one area and thinned in another. Note which is which and decide from the results what you prefer. 

You are going to apply this gloss coat with a brush and not by airbrush you say?