ve heard of this but i cant find a menu in gtk with how to…
heres a screen

The screenshot wasn’t really necessary, we know how GTK looks 
But yeah…
Right click in one of the 2D windows (upper right, lower left or lower right) and choose “misc” then “misc_model” or “misc_gamemodel” and then choose your md3 file.
After that is done press N to bring up the entity window.
The path after “model” will probably look something like
“C:\Program Files\Wolfenstein - Enemy Territory\etmain\models\yourmodel.md3”
Change this to
“models\yourmodel.md3” (delete everything infront of “models”) to see the model in GTK.
BUT
Next time please use the SEARCH button because this question is so trivial it will have been answered 5000 times before.
right click the 2d view window, select misc, misc_model, find your md3 file and submit.
// edit:
ffs, i was slower -_-