do you use it in gm_construct's color room or on gm_black? or this happens anywhere

I think it happens on gm_black, because i had the same issue. Weird thing is it works sometimes, but after a few clicks stops again.

put up some big props like that plate from building tab around your "working area" temporarily, so raytracing stuff won't go through gizmo into whatever brush entity that's gm_black's floor. dunno why but any toolgun thing doesn't quite work with it