i found the solution on gta forums, and it worked for me.. check it out:
Guys guys...I just figured it out how to make the analog triggers work!
Just search your registry for the joypad name appearing in Game Controllers in controlpanel. You'll end up at some key similar to:
HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Me diaProperties\PrivateProperties\Joystick\OEM\VID_0 6A3&PID_F51A
Next to OEMName will be the name of your joypad. Right click "OEMName" select modify Enter (exactly as here):
Make sure that the name changes in Game Controllers in control panel.
Now start the game enjoy analog triggers!