The Z-angled spacers control how close the v-slot wheels are to the vertical rails that the X-axis arm moves along. Adjusting this spacer is part of general maintenance for your gMax. Making sure the wheels are in contact with the vertical rail will ensure smoother movement in the z-axis
If the v-slot wheels are to tight on the vertical rail, you may have problems with the z-axis binding up and not raising properly. You just want to make sure the wheels are snug to the rail, and moving when the x-axis arm moves up or down.