Why do I get an open ground warning after a GFCI Test?
sounds more like something is wired wrong. The test button should drop the hot, not the ground. with the hot out, your tester would be dead all 3 lights. The black wire should go to the brass colored screw. The white wire should go to the silver screw. The bare copper should go to the green screw. these are on the feeder side, the wires to any other outlets follow the same color code but they should be fed from the bottom looking at the back of the  GFCI outlet.