Changing it through memory would require modifying the following:
vehicle.cc
const F32 sVehicleGravity = -20;
wheeledVehicle.cc
static F32 sWheeledVehicleGravity = -20;
player.cc
F32 Player::mGravity = -20.0f;
item.cc
F32 Item::mGravity = -20;
debris.cc
Point3F force = Point3F(0, 0, -9.81 * mDataBlock->gravModifier );
.. and so on and so on. There is no simple, clean way to change the gravity for all players, vehicles, items, projectiles, etc.