First I have to say, that i'm a newbie at GTKRadiant and that i'm a german. Sorry for my strange language.
I downloaded GTKRadiant 1.5 and Nexius. Then i saw this post and downloaded the Zip by tZork. I know it is for GTKRadiant 1.4, but here is my newbie-solution to work with 1.5.
I don't know if it is usefull, .. test it out
Run the program of tZork and try to install it.

But the .exe is named GtkRadiant.exe and not GtkRadiant-1.4.0.exe, so renamed it to GtkRadiant-1.4.0.exe and the installer of tZork works. (Rename the exe back to GtkRadiant.exe after it)
When you now start GtkRadiant.exe, there will be many errors... well I waited for them...
open the nexuiz.game in the games folder of GtkRadiant and edit it like that:
<?xml version="1.0" encoding="iso-8859-1" standalone="yes"?>
<!-- generated by Radiant setup, modify at your own risks -->
<game
name="Nexuiz"
enginepath ="C:\Spiele\Nexuiz\"
enginepath_win32="C:\Spiele\Nexuiz\"
engine_win32="nexuiz.exe"
gametools="C:\Spiele\nexuiz\Radiant-1.4/"
basegame="data"
engine="nexuiz.exe"
gamename="nex"
basegamename="Quake III Arena"
knowngame="missionpack"
shaderpath="scripts"
archivetypes="pk3"
texturetypes="tga jpg"
modeltypes="md3 ase lwo obj 3ds picoterrain"
maptypes="mapq3"
shaders="quake3"
entityclass="quake3"
entityclasstype="def"
entities="quake3"
brushtypes="quake3"
patchtypes="quake3"
/>
open the local.pref file in your Nexuiz\Radiant-1.4 and the same file in GtkRadiant 1.5.0\nexuiz.game. Edit the following entries.
<epair name="LastProject">
Copy the default_build_menu.xml files form the q3t.game folder to the nexius.game folder and edit
[RadiantPath]: path to Radiant ..
[EnginePath]: path to the engine ..
So that was ist. After the modifcation of the stuff above GtkRadiant started on my computer without errors
P.S.: perhaps you can edit the local.pref files and the nexuiz.game before you install tZorks program, you only have to edit them in the unzipped folder.
If there something wrong please tell me.