how do you add new textures to gtk radiant?

Textures should be .jpg or .tga images, with dimensions that are any combination of powers of 2 (2, 4, 8, 16, 32, 64, 128, etc). Common examples are 256 x 256, 512 x 256, or 64 x 128. Make a new folder in your GameData\base\textures\ directory, name it whatever you want your texture set called (eg, "mymap"). Put your new texture images into this folder. Then look in the Radiant Textures menu, you should see your folder name ("mymap"), click it, and your textures will load into Radiant!

ALWAYS remember to put your custom textures into the .pk3 file when you finish your map. In fact, it's a good idea to put them in there when you start your map. They go in the textures\ directory inside the .pk3.