I figured out my problem. I was working off of Sandy's $RADIANT/games/trem.game file, and it was slightly wrong, at least for 1.4.0.
Here's my guide. I'm running Slackware 10.2, Tremulous 1.1.0 and GtkRadiant 1.4.0.
I'm going to use $TREM for the base install of Tremulous, and $RADIANT is the base install of GtkRadiant, my defaults are /opt/tremulous and /opt/GtkRadiant-1.4/, respectively. In the file listings below, I'll have copied and pasted them from the real deal, so they won't have $TREM/$RADIANT in them, you'll just need to do a global replace.
First, install GtkRadiant and (presumably you've already got Trem installed, if not, install it--I'm doing this on my laptop, which didn't have Trem installed, so it's possible). When you install GtkRadiant, install at least the Q3 datafiles, you'll need these to get the default project files (this is even if you don't actually have Quake3 installed, again, I'm working on my laptop which is not my gaming system).
To fellow Slackware users: I had to install gtkglext, as it didn't come with the base Slackware install (Slackware includes minimal Gtk stuff, since they don't package Gnome). Other distros: yours probably comes with gtkglext.
Next, you'll need to grab
this zipfile (it's the same one listed in an earlier post). It has entities.def and notex.tga in it (notex is in the common-spog.pk3 file). Unzip this into your $TREM/base directory--you should have $TREM/base/common-spog.pk3 and $TREM/base/scripts/.
Now, to tell GtkRadiant that Tremulous exists. You need to create the $RADIANT/games/trem.game file. Here's the one which works for tremulous and GtkRadiant 1.4 (the file uses my default paths):
<?xml version="1.0"?>
<game
type="q3"
index="1"
name="Tremulous"
gametools="/opt/GtkRadiant-1.4/trem/"
enginepath_linux="/opt/tremulous/"
enginepath="/opt/tremulous/"
engine_linux="tremulous"
engine="tremulous"
prefix=".tremulous"
basegame="base"
basegamename="tremulous"
knowngame="base"
knowngamename="tremulous"
unknowngamename="Custom Quake III modification"
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"
/>
The next thing I did was fire up GtkRadiant and start up using Quake3. This generates the file: ~/.q3a/baseq3/scripts/user0.proj Open this up and run two global replace operations:
search for: usr/local/games/quake3
replace with: $TREM
search for: baseq3
replace with: base
Under vim, on my system, this looked like:
:%s/usr\/local\/games\/quake3/opt\/tremulous/g
:%s/baseq3/base/g
You need to save the modified version to ~/.tremulous/base/scripts/user0.proj (you'll probably need to create the ~/.tremulous/base/scripts directories).
Next time you fire up Radiant, use the "tremulous" option on the "Select Game" dialog box, and map away.
Jeff