Generating the end caps of a cylinder using triangle strips for use in a geometry shader
If the end of your cylinder is convex, with vertices $V_0, V_1, V_2,...V_{N-1}$ then just replace your fan order (i.e. as above) with this strip $V_0, V_1, V_{N-1}, V_2, V_{N-2}, V_3...$, i.e. just zig-zag across the polygon.