gloss or semi gloss?

It sounds like the decals were poorly bonded to the gloss coat underneath. When the gloss coat on top cured, it shrank (all coatings shrink, at least a little, when they cure). The shrinkage caused the decal to debond and crinkle.

Not all gloss coats are created equal. I, too, recommend Future as the "safest" for the decals, underlying, and overlying paints.

Put a gloss coat on before you decal, it will help stop the decal from "silvering". If you use a semi-gloss coat then you will still get air bubbles under your decals and will cause the dreed "silvering" effect.

As far as the decals crinkling, it depends on what you are using for your gloss coat. A great thing to use for a gloss coat is Future Floor Wax. You have find it in any grocery store or other stores that sell house hold cleaning products. You can either brush it on or spray it and you don't have to thin it when you do spray it. I highly recommend using the Future, I have used it for over ten years and have never had a problem with it.