To get weapon swaying into Garrys Mod (the effect where the weapon shifts back and fourth" follow these 3 steps.
- Open your console.
- Type in “cl_wpn_sway_scale (how much swaying you want, default is 0.5)”
- Type in “cl_wpn_sway_interp (the timing for how long the weapon returns to normal position, default is 1)”
That should do it.
Note that it will look somewhat wierder then in Half-Life 2. This is because Garry sped up the weapon animations. Making the effect un-noticeable without using these console commands.
(This also works with almost any other source engine game.)