Any gap can be a problem. You want to make sure that the wing joiner is well-glued in its pocket, and that the ends of the wings that butt together fit with zero gap! If you have a gap, then that means that the wings are not well-glued. The wing joiner does not usually carry 100%of the loads across the joint, but needs good gluing of the spar-ends as well. Also, if the wing has a gap in the center, then there can be some movement between the wing panels that can cause the center joint to fail. People who have this happen usually think it's a problem in the quality of the design or construction when it's really their assembly job.

If the wing panels have a gap, then sand the ends of the joiner to fit. If there's a gap on one side and not the other, sand the ends until you have a perfect fit. The better fit you have, the stronger the wing will be.