Oh, buddy that's not a ghr if your knees stay flexed the whole time...it's just a hip extension with flexed knees. For a ghr while the hips are flexing the knees have to extend and then while the hips are extending the knees flex. So it's simultaneous hip and knee flexion which is working both functions of the hams and the glutes as well.
Don't get me wrong..keeping you knees flexed against resistance like that will work the muscles more but it's not a good thing to do, I don't thing and it may, if overdone, encourage some shortening.
That's actually not to bad an idea...