Well, my advice would be to go buy a 10 gallon qt tank, but if you cant do that there are a number of options, i personally dont like to medicate my tanks, unless there in a qt tank, so what you cn do and i have found to work really well is to make up a batch of freesh water that matches temp and ph of your salt water and to do a fresh water dip. these can be tricky however, the fish should be dipped for between 3-8 min depending on the fish, ask your lf how long they think it should be dipped for, you can find more info on these dips online, there might even be some on this site, also you Royal Gramma prolly already had ich when you brought it back from the fish store, good luck
Brahm