yes they will fit.
all 15s are the same size on the inside. and even though the GT came with 16s, that is mostly just for show. the brake set up is the same as the outback and the suspension set up is the same as the legacy and impreza.
as long as the tires fit the impreza, they will fit the GT.
but you might google a ''tire calculator'' so you KNOW your speedo error. you will probably be traveling faster than the speedo says.