There were users who used Gig Performer with 85% of RAM used. Personally, I wouldn’t go any further (say 90% or more). Keep in mind that if your Windows computer is not prepared (i.e. optimized) for a performance that other processes may run automatically and interfere with your setup and performance.
Gig Performer will not crash, but you’ll most likely hear glitches due to transferring data from RAM to the Page File.