for GTKRadiant all you have to do is the following:
in your gtkradiant install directory copy over /games/q2.game to /games/pb2.game then open the pb2.game file and edit the paths to your paintball2 game. my pb2.game looks like this:
<?xml version="1.0"?>
<game
type="q2"
name="Paintball II"
enginepath_win32="f:/games/paintball2"
enginepath_linux="/usr/local/games/quake2/"
enginepath_macos="/Applications/Quake2/"
engine_win32="paintball2.exe"
engine_linux="quake2"
engine_macos="Quake2"
prefix=".quake2"
basegame="pball"
default_scale="1.0"
no_patch="0"
no_bsp_monitor="0"
/>
Then copy over the q2.game directory (in your gtkradiant install dir) to pb2.game
Next you need to put the PB2 entities into gtkradiant, so open {$your_gtkradiant_install_dir}/pb2.game/baseq2/entities.def
and erase anything in there and replace it with what you can find in bsp/pball2/entity/pb2ents_b5.c (when you download bsp optimized for pb2 which you can find in the files section)
That's all, just restart gtkradiant, select the Paintball2 game, and off you go.