if you want a serious answer, ask a lawyer.

you wont get an answer that will stand up in court here.

source: http://www.flipcode.com/archives/Quake_2_BSP_File_Format.shtml

Quote:
Legality


Using id Software file formats in a publicly distributed application always seemed a little questionable to me in terms of legality. In preparation of this document I contacted id Software to resolve the issue; John Carmack was kind enough to send along this response:

"We do not legally protect the file formats, but you can't use any of the released tools, or tools derived from them (except the GPL'd Quake 1 tools) to generate content for a commercial venture. If you are writing everything yourself, no problem."

You could always email id or unreal to double check.

GTKRadiant is based off the QERadiant(developed by ID for Quake 2 and onwards) editor at least as far as design. I do not now if it is based off QERadiant's source code however. The map compiler tool(which turns the file the editor saves in to the format that the game engine uses) however is the one developed by ID so that you will not be able to use.

Wolf_Tool is developed by Splash Damage and it was custom designed to work with Enemy Territory so that's pretty much another no go.

You might try looking into QuArK(Quake Army Knife http://quark.planetquake.gamespy.com/). It's a bit different than Radiant but has a pretty strong following from the mappers that use it so tutorials and advice should be fairly easy to come by.

As far as I can tell you're still going to have to write your own compiler to use these editors in a commercial project. On the other hand you won't have to necessarily use the bsp format which may throw in a lot of stuff is unrelated to your game.

For actually drawing the levels this will help you get the geometry up and running, however it does not deal too much with how to use the Potential Visibility Sets for geometry culling. It however links the Q2 flipcode tutorial on helping with that part.