I know, that's why I corrected you changes in gravity affect your weight NOT your mass. On the moon you have the same mass, different weight.
Szeth never builds huge momentum and then become weeightless, the closest I can think of is when he half lashes a table upwards and then kicks it towards some soldiers, and it clearly has an impact. Stopping gravity's effect on you won't change the momentum you already have, although you could use it to slow down. Eg. If halfway through his fall Kaladin lashed himself upwards he would have decelerated and been travelling at 0 when he hit. But he doesn't slow or stop until right when he actually hits the ground, which means he either instantly lashes himself upwards multiple times (Like 20 or so) then back downwards or he has used a separate ability and again Kaladin tried to do a Basic lashing only a few minutes before this and failed. And again doesn't explain the ring of Stormlight that hits the ground.