Enable Cheats
Once in the game, open the console by holding down the "Tilde" (~) key. This brings up a menu which allows you to type in commands. Type "sv_cheats 1" and hit "Enter" to enable cheats.
Adjust the Gravity
Open up the Counter-Strike program and enter the server. Then press the "~" key. This will open up access to the game console. Change the gravity settings with the command "sv_gravity" followed by a number. It should look like this: "sv_gravity 800".
Experiment
Play around with the server setting until you find the desired outcome. The default setting for the game is usually around 1000, so if you use that as a benchmark, you shouldn't have much trouble finding your desired setting.
Other Options
There are other ways of altering the gravity settings, but these are hard-coded into the game's CFG file. Under no circumstances should you alter these settings unless you really know what you're doing. Otherwise, you run the risk of corrupting the game data and rendering your copy useless. Stick with the aforementioned method.