GtkRadiant can not open the .bsp, but you can use q3map2 to convert an existing .bsp to a .map file.
"/opt/gtkradiant/q3map2.x86" -convert -v -game tremulous -fs_basepath "/opt/games/tremulous/" -fs_game base -format map thebspmap.bsp
This command will convert thebspmap.map into thebspmap_converted.map, which can be opened in GtkRadiant.
You could use this to figure out how a map author created certain effects, put please do not use this for blatant copy-pasting. Keep in mind that the result will not have any light entitities.
The snippit is a linux command line and you'll have to edit it to use it on windows (or your particular linux installation, for that matter). Tested with GtkRadiant 1.5 and q3map2 2.5.16.