GtkRadiant 1.5.0 is still under developement. The latest nightly build is available for download in RPM format here .
The nightly build tends to be outdated, so you can also try building the source. Instructions for doing so are at GtkRadiant1_5-Linux-Compiling .
Since its an RPM file, the installation process would change depending on what distribution you are using:
For RPM based distros Here.
For Debian based distros Here.