cesege
Again, welcome to the forum. I'm not sure about poulder, but I'm assuming its like SNJ, that is you apply it to the area you want it with a soft brush or q-tip and then polish it to the sheen you want with a cotton ball. If this is the case, you should have no problems applying a gloss coat over it.
My suggestion would be to take an old model (or buy a cheap model and assemble it) and try what you want to do on it. That way you can find what works with out damaging your Gloster. Word of caution on aluminum paints (whether applied from a rattle can or air brush), you will find that almost all aluminum paints (except for Alcad II) will lift if masked over with regular masking or modeling tapes. To overcome this (especially on those panels that you are applying the poulder to) you can mask with Post-it Notes or damp newspaper, both of which work very well and won't lift the aluminum paint under them.