You must have the Logitech software installed.
Go into Wingman Profiler
Click Profile (in upper left)
Select New
Select Create From Scratch
Click Next
Name the profile (GTA3 or whatever), click Browse and find the GTA3 executable and select it, then choose your icon, click Finish
Click 'Right Stick X Axis' and select 'Axis Properties'
In dropdown box at bottom where it says 'Right Stick Axis mapped to', select Throttle (at top of list)
Click OK
Now to play with new mapping, you must click "Play GTA3" (or whatever you named it) from within the profiler. You can adjust your other stick, buttons, controls, etc from within the profiler as well.
BTW: You could most likely get more in depth with the settings and fine tune them, this was just simplest way I could think of to get the effect you were looking for...