GtkRadiant is now installed and should work without a problem for Quake 3, however we are here for Quake! (or Tenebrae or any other Quake based modification)

NOTE: If you do not have file extension enable in Windows Explorer you may have a hard time with some of the following steps, so lets fix it. In Windows Explorer, select Tools -> Folder Options -> View and make sure you have Hide extension for known file types un-ticked.

First thing to do is make a quake.game file. Navigate to d:\GtkRadiant\games and make of a copy of q3.game (Select it, then press CTRL+C then CTRL+V). Select the file Copy of q3.game, press F2 and rename it to quake.game. The .game files are really plain text xml files and we need to edit the new quake.game file. Open quake.game in Write.exe or Notepad.exe (click on it and select Write or Notepad from the list that Windows offers).

Remove everything, then copy and paste the following into the file:

<?xml version="1.0" encoding="iso-8859-1" standalone="yes"?>
<game
  name="Quake" 
  gametools="D:\Quake\Radiant/"
  basegame="id1"
  engine="fitzquake065.exe"
  default_scale="1"
/>

The line that starts with gametools= needs to be where you have Quake installed. The '\' and '/' are correct even through it may look wrong. The basegame= line is where GtkRadiant will start to look for certain files. If you are installing GtkRadiant for Tenebrae only, change this to tenebrae. The engine= line is not really required but make sure you change it to something that you have in your d:\quake\ folder. winquake.exe, quake.exe, glquake.exe are all fine. default_scale= tells GtkRadiant what scale to use when mapping textures to brushes. For Quake editing a scale of 1 is better than the default 0.5.