You need to install 1.1 before GPP. If you installed 1.1 from some repository, I recommend downloading the 1.1 .zip from tremulous.net/files instead, extracting it somewhere, and extracting the GPP .zip in the same place, so the binary files end up next to each other. Then open a terminal and type:'/path/to/tremulous-gpp.x86'
If it still doesn't work, post the Terminal output.