There is no reason to leave a gap. Drywall sheets now come with a beveled, or "scooped out" long side that mates up to a corresponding beveled edge of the next sheet. They do this so you can lay in mud, tape, and more mud and not worry about creating a hump in the wall. This will work well for you.
I'm about to drywall my basement as well, but only the top 4 feet of the walls. The bottom 4 feet is brick, and I don't want to cover that. The problem I'll run into is making the vertical seams between sheets look good...since there is no bevel on the short ends, as far as I know.