Have you tried the coke bottle trap? This is an easy to make, reasonably effective trap which I imagine should work well for a fish like a royal gramma. Bait it with some sort of meaty food.
Unfortunately, if the royal gramma has ich you have to assume all your fish have ich. You may have to dismantle your live rock, etc, to be able to catch and treat all of your fish. That's what I had to do when I dealt with ich. It was quicker to just do it than to agonize about having to do it and trying to catch wary fish with a net.