Im using GtkRadiant 1.5.0 i want to convert a .BSP to a .MAP file
Uhh, Ever tried google? you know, one of like the first results you get there answers it... <.
Decompiling maps really shouldn't be used without the approval of the original author of the map. Not even mentioning redistributing said maps.
Decompiling also deletes all Md3's and mis-aligns the textures of the map. It is also almost impossible to recompile said maps because of so many errors. I suggest not even trying to decompile, it is too much trouble.
You should ask the original author's permission for the map source files, it's way easier. You'll lose lighting, texture positioning, some brushes, terrain metashadering and some more if you decompile a bsp.