[Babul]

walls below a size of 1mІ (meters or units, its the same in GTA) cant be detected by the mapandreas plugin, its resolution is too low, and it wont detect custom objects. if any object is bigger than that mІ, like a house, sometimes a plant/tree, or simply a canyon, can be detected.
to apply the (fake) gravity can be easy:

pawn Код:

new Float:ZVelocity[NAX_GRENADES];

then, in a really short timer which loops through all grenades, increase the ZVelocity[grenade] about 0.01 and add this ZVelocity to the objects current position (even while its moving, the new speed will get applied to the grenade).