Make sure the offset you are modifying, is the correct one, if you are unsure, check the thread posted directly below:
Modern Warfare 3 Source Code / Address Thread
This thread should contain offsets and bytes for all versions of "Call of Duty®: Modern Warfare® 3", which should be
Steam, Tekno, or IW5M (fourDeltaOne).
P.S.: There may be other versions / modifications of MW3 that I am unaware of, if
there is, consider that it may be in the thread linked above.
If you still need help, please mention my name in a reply (@LEGiiTxCHAOTiiC) (otherwise I probably will overlook this thread), and state which version of MW3 you are planning on doing this modification on, whether it is Tekno, Steam, or fourDeltaOne's IW5M, respectively. Please note: I cannot find the MP versions of these offsets for Tekno or Steam, as I only play IW5M. For SP offsets, again I can only find on IW5M SP (which I believe is the same version as Steam, but I'm not sure).
Default value for gravity: 800 (Hex: 0x320 / 320h)