Hi, If the board is big enough and not secured to the cabinet find the largest gap between the board and the sheetrock. Say for example the gap is 1/4 inch. Get any thing 1/4 thick and about 6 inches long. Go to the end of the board and hold the 1/4x6" piece flat against the wall tight and with a pencil use the 1/4x6 inch as a guide mark the board from one end to the other end. When you have the mark it should be 1/4 where the board is tight at the wall and less where the is a gap. Now plane or sand your board to the line. If done right the board should fit against the wall

Good Luck Woodbutcher.

Or if you’re not going to move it, caulk it.