Gtkradiant doesn`t have a textures menu like Alien Arena`s Q3 Radiant
my questuion is this how can add third parties textures in Gtkradiant
Thanks!

Drop them textures into the Nexuiz>data>textures folder. If you dont have one, make one. All there is to it.

Then to add them on somthing, View > Suface Inspecter or 's'