I get what you're saying, but I'd personally rather have a GSSF membership in addition to a Glock. If you'd rather just have the Glock, then by all means do just that. This isn't really a hard concept to figure out.
A brand new G19 is $499, with taxes you're looking at $540.
A 2 year membership with GSSF, which is what you have to start off buying if you immediately want that coupon, is $60.
The discount G19 is $425, with taxes it's $460, include the membership and you're in $520. You've save $20 on a new Glock, and you now have a 2 year membership with GSSF.
One calendar year from that point, you'll be eligible for another discount Glock. Let's say you get a G17 now, that's another $425; $460 with taxes. If you had bought another Glock that following year you would save $80 on that purchase.