is it possible to select half a map in gradient i think i saw wyat do somthing simular on one of his tutorials i want to delete half of it

for cod radiant all you do is create a brush around the half you want to get rid of. The brushes to be deleted would be inside of this large brush. Then you right click/select/inside.

NOW it will not only miss some of the brushes that are not fully inside that brush but it may also select ones you don't want to delete so before you hit delete check the brushes over to make sure you want to delete them.

might be a good idea to make a backup point of your map before and after you delete them just incase.