I know of no way to paint a "decal" since what you have is really a vinyl sticker. Paint just doesn't do well on them.
But you can paint the tailgate first, apply the stickers and then go over the tailgate with a clear coat. The danger is the clear might cause the stickers to lift at the edges before the clear dries. If you do this, apply a thin coat of clear first, follow with another while you can still re-coat but toward the end of the re-coat time range and then let that dry. Say a prayer the vinyl doesn't lift. I've seen it happen. Lightly sand in 3 days and apply a heavier coat of clear. Then wet sand with 1000 grit and polish using 3M 05954 or whatever rubbing compound you like. If you do it right, you won't even feel an edge there. Good luck.
