Have no clue what happened to my GIMP installation. Every time I am using select tool (any - square, round, magic wand, etc) - it just freezes whole Gnome Shell. But I still able to get into VT with Ctrl+Alt+1 and kill process from there.
After googling everything - nothing came out and I tried simply delete ~/.gimp-2.8 folder. This did it! Probably some settings somewhere made GIMP get into forever loop or something..