In case it's still not clear for some people (it sure wasn't for me):
You need exactly three files: data-gpp1.pk3, vms-gpp1.pk3 and either tremulous-gpp.x86 or tremulous-gpp.x86_64 depending what system you run. You put the pk3's in a gpp/ dir near a base/ dir from 1.1 (either in ~/.tremulous or wherever you got the main files, I'm on Ubuntu). You put the executable in the same dir with the base/ dir. You run the executable. That's all.
For clarification, here's how it should look:
base/ <-- all the 1.1 stuff
gpp/data-gpp1.pk3 <-- 1.2 GPP data file
gpp/vms-gpp1.pk3 <-- 1.2 GPP data file
tremulous-gpp.x86 <-- 1.2 GPP executable (32 bit)
tremulous-gpp.x86_64 <-- 1.2 GPP executable (64 bit)
In addition, the dev servers may push to you updated versions of those pk3's (such as data-gppr1980.pk3 and vms-gppr2006.pk3). Those will go to ~/.tremulous/gpp/.
PS: Devs, it would be really helpful if you provided a ZIP with just these four files and a README.txt. Some of us like to know what's going on, rather than trusting an install kit, however trustworthy the source of that kit may be. Besides, my /home is noexec, placing executables in my ~/.tremulous dir is useless (just goes to show one size does not fit all).