I assume that what you mean by “gap” is margin? As a rule, I paint all works on paper with a 3/8″ to 1/2″ margin. Also, I make a point of avoiding painting anything precious too close to the outer edges. I’m also a custom framer and this habit makes my life as a framer much easier. Since watercolor paper, mat board, glass and frame all expand and contract at different rates, a crucial part of framing is accommodating these dynamics. That means a healthy amount of overlap of the mat over the edges of the art. Expansion and contraction can cause intermittent exposure of the margins within the mat opening if there’s no overlap. As a rule, I like to have at least 1/8″ of the painting go under the mat opening all the way around.

Give up on trying to eliminate or minimize overlap. It will drive you crazy.