The gloss coat needs to go on first to bring the sheen of your paint up to the same gloss as the decal film. then decal then clear coat or dull coat for the look you want. if the paint is not as shiny as the clear part of the decal you will not be able to hide it.
Not true. The gloss coat is put on first in order to help prevent air bubbles under the decal. If you don't get all of the air out from under the decal, it will look like silver spots. The surface of flat paints are not as smooth as the surface of a gloss or satin finish paint. The smoother the surface, the easier it is to squeeze the air out from under the decal. Decal setting solution helps let the decal lay down over surface details.
Before you guys said that a gloss coat was needed, I never used a gloss coat. I decaled right over the flat paint and never had any problem hiding a decal. Of course, what brand (read thickness) the decals are make a big difference.