GtkRadiant simply does not support editing BSP format (well it's rather IBSP) as this format was not designed to be modified in such manners. BSP is already baked-in geometry with lightmaps (these are images that are used to enlight the map geometry including brushwork entities and models) and this format is binary (which means that everything you see in game are only numbers that are converted to actual graphics data by the game render). For a map creator it's much easier to work with text format, which is the format that GtkRadiant use for the *.map files. From map files you can make bsp using a compiler, which is generally converter that makes the text-based map format into the binary IBSP data. The proccess of that conversion is called "compiling" and it consumes a lot of the calc power to actually calc the geometry, the vis and the lightmaps. Bigger map means longer compile time... That compiler is a program - it's called q3map2. It's a command line only utility - it reacts only to given commands with parameters required to actually compile the map. You can read further details here:
https://en.wikibooks.org/wiki/Q3Map2
To the bsp2map converter - there is already one: it's the same program used to compile bsp files (q3map2), but instead of actually convert text-based map file into bsp, you should flip that proccess and convert bsp back to map file instead. That flipped proccess is oposite to the "compile" term - it's called "decompiling". You can do this by adding the parameter
-convert -format MAP after the q3map2 command... That way you can successfully convert already compiled BSP into the workspace text-based format MAP. Basically - each computer algorithm can be flipped to the opposite, so if the program can actually compile, it should be able also decompile. Like when you're trying to convert one image format to another and vice versa. It's the same principle
There is also analogy and might help you to understand the basis: GtkRadiant works always only with MAP files as it understands basic text-based code and cannot load up BSP's - Quake 3 game engine can't load up workspace text-based MAP files as it understands only the binary format IBSP (the BSP files are this format). To load the map in each other engine (editor or game), you need to convert that map using the computer algorithm actually calcing the values required to be assembled for the required kind of engine...