Remove the damper and spring and fill the fork with diesel or gas to thoroughly clean the insides out then spray WD 40, liquid Wrench or other easing fluid inside the fork and work it in and out until it gets easier to move, then mount it back in the triple clamps or well protected in a sturdy vise and use a rubber mallet to hit the top of the fork sliders downward. Just to get it started If you leave the seal retaining collar in place but loosened by a couple of turns you might try a plastic, leather or even copper mallet if you are careful not to damage the fork tubes, maybe wrap some masking tape around the tubes before administering the beating.
As an addendum, during disassembly heat up the lower legs with a propane torch but be careful of the potential fire-hazard of all of the volatile fluids that you have just used for cleaning and lubrication!!!!