What game? Engine? We need a little more info.
Essentially, this is what I think is happening, but I haven't really sat down and thought about it thuroughly.
On the line, right above where the arrow is pointing, you assign a static variable dword_10503540 to point to the address of a property, "PlayerGravity". If that property doesn't exist (i.e, that pointer is null) you create it and assign it the value of -9.8 (that is the acceleration due to gravity.)
So, what might work is to change the -9.8 to -2.0 etc (which will reduce gravity's acceleration) or 0, which will negate its effect. That said, it may not work if this property is initialized elsewhere (You could follow the thiscall to figure out where if this does not work.)