you can change gravity on a standard server, no mods.. fall damage you cant tho
g_gravity 800 //800 is stock, fun comes at 50
set ex_falldamage_enable 1 //1 is on, 0 is off
set ex_falldamage_min 15
set ex_falldamage_max 30
correct me if im wrong, but it is in x4?