What is the difference between Gyrotonic® and Pilates?

They both are core strengthening, postural correcting, alignment focused movement systems.
Gyrotonic® moves in a 360 degree plane – arch, curl, side bending, spiraling, and undulation. Pilates tends to be a little more linear in nature, especially for the beginning student.
Another difference is the equipment – Pilates works with springs that extend from a fixed point while Gyrotonic® equipment uses weights connected to cables that feed from a pivoting reel and their tension does not increase as you extend in all different directions – unlike springs that increase in tension as you press out. This creates a much different feel – the eccentric part of the movement, especially.