You'll receive better stimulation using a barbell.
Stiff legged deadlift will more efficiantly and safely strengthen the leg bicep.
If you have poor strength you will need to bend the leg slightly, but with continued use you will be able to properly perform this exercise with straight legs while maintaining your spines natural curvature.
The GHD will only apply direct resistance for roughly 150mm over 800mm, for me, this a piece of equipment which is equivalent to a smith machine and probably have ruined 1000 knees for each one they've helped.
But this is just my opinion and opinions will vary.