How is it perpetual motion if all you are doing is blocking the effects of gravity?
If you could block the effect of gravity, you could use it to lift a large weight with a small amount of energy, after which you could turn off the device, and capture the energy as the weight fell back down. This cycle could be repeated indefinitely, yielding inexhaustible free energy. That energy could in principle be used to power the device, hence perpetual motion.
"How is it perpetual motion if all you are doing is blocking the effects of gravity?"
It depends on how much energy it takes to operate. A rope and a pulley attached to an overhead beam is an "anti-gravity" machine. You pull on the rope, raising a weight, and then let the weight fall down and exert force on an object below. But the net energy expended to raise the weight is less than what you get out of dropping it, due to friction and other losses. If the anti-gravity device could raise the weight with less energy in that you get out of dropping it, then you could easily build a perpetual motion machine.
Of course, if step 1 takes a hundred times more energy than you get back in 4, it doesn't yield net work. So the statement is true if and only if the alleged gravity stopper doesn't care about energy costs every much.