GtkRadiant reads and writes game assets from within your game's directory. Before you start installing GtkRadiant, first make sure you have the game you want to edit for installed. It is recommended to run a "clean" install: creating a separate installation of the game free of any third party custom content.
Recent versions of Windows (starting with Vista) consider certain directories as "protected" directories, meaning you don't normally have write access to them. For security reasons, this is a good idea since it prevents rogue software or malware from installing stuff in places it shouldn't. However, this affects GtkRadiant's ability to write within those directories to save your map files. Games may default to installing in "C:\Program Files\" or "C:\Program Files (x86)\", both of which are write protected. It's a good idea to change this to somewhere else like, "C:\Games\".
GtkRadiant 1.6 is packaged as a compressed zip file. Extract it somewhere on your system such as C:\GtkRadiant. Double-click on radiant.exe to start GtkRadiant, be patient while the program initializes for the first time. The game selection menu will appear. Select the game you want to configure, and choose the engine directory of your game (where the game .exe file is located), and hit OK.
GtkRadiant 1.5 is packaged as a single installer, regardless of what game you are editing for. Just run the installer and follow the instructions.
The SDK files are packaged with a separate "gamepacks" installer that you will need to download and install after you've installed the main GtkRadiant program. Follow the instructions and locate the game directories when it asks you.
GtkRadiant 1.4 comes in several different prepackaged installers, complete with the game SDK. Make sure you download the right one for the game you are working on. For example, the installer "GtkRadiantSetup-1.4.0-Q3RTCWET.exe" installs GtkRadiant and the necessary SDK files for the games, Quake 3, Return to Castle Wolfenstein, and Enemy Territory. If you are working on Star Wars: Jedi Academy, you'll need a different installer.
Clicking on the executable, GtkRadiant will walk you though the installation. It will ask you to locate the installation directories of the games that you will be editing for. Browse through your system directories to locate them.
Windows Desktop Composition
The Windows Vista and Windows 7 desktop theme may not work well with GtkRadiant on certain system configurations. After installation, right-click on the GtkRadiant.exe and choose "Properties". Click on the "Compatibility" tab and under "Settings", check the "Disable desktop composition" option. Click on "Apply" and "OK".
Whenever starting up GtkRadiant, Windows will disable the desktop theme and start up normally. Don't be alarmed if a few extra Windows features are disabled, like taskbar window previews. Close GtkRadiant and Windows will go back to the default desktop theme.
Run as Administrator
If you are running Windows Vista or newer, and GtkRadiant 1.4 or 1.6, and have installed the program in "C:\Program Files", the program may have issues saving preference files, logs or other program files. This is a known issue and is being investigated by the developers. The workaround for this issue involves starting GtkRadiant with elevated privilages using the Run as Administrator option in Windows. You can find this option in the same "Compatibility" options as the above mentioned "Aero" fix.