For decals most people spray a gloss coat before the decals then a matte or semi gloss when every thing is done. You dont want a shiney gundam. The gloss coat is supposed to make it easier to apply decals but if it's not painted you probably dont have to worry about that. I bought a can or Mr Hobby semi gloss top coat off of ebay and I think it looks great. Might want to avoid spray from the store since some of the chemicals might have a reaction to the plastic or decals.
Have you tried panel lining yet? It really makes your kits pop. I just got a gundam panel line marker but you can get a super fine marker from an art store by Micron. If you decide to do that before you do the top coat.