The
GSSF Registration Form is a little easier to follow than the rulebook for seeing which models are allowed in which divisions. So you'd pay a separate entry fee, get separate score sheets, and just shoot the same gun another time for each additional division.
The 21 could be a good choice, Heavy Metal is a great division to shoot. If GSSF is your focus, the dark horse candidate would be the 30...gives you access to Subcompact, Heavy Metal, and Major Sub...everything the 19 can't do (well, aside from Pocket). With the standard finger-rest 10-round mags, it is identical in grip height and sight radius to a 19, just in .45 ACP, and is surprisingly easy to shoot especially with lighter loads.