Okay, here's one way you could do this.
Game Syncs Tab (Screenshot: Game Syncs)
You will need a switch group. I'm going to call it SG_Gears. It will have 7 switches for each of your gears including park, and you need to check "Use Game Parameter" in the Switch Group Property Editor. Choose "Throttle" as the game parameter to use in the bottom right of the same editor pane. Now create points on the parameter graph that correspond with the gear changes you want to make.
Audio Tab (Screenshot: Switch Container, Sequence Container)
At the top, you will need a Switch Container. In the Switch Container Property Editor, set the play mode to continuous. Set the switch group to "SG_Gears" and set the default switch to whatever gear your player starts in.
The switch container will have 7 Sequence Containers as its children that correspond to your 6 gears + park. Set each sequence container's play mode to continuous as well.
Each sequence container will have two Sound SFX objects as their children: one with your gear switching sound, and one with Wwise Silence (or just a .wav of digital silence). The silence objects should all loop infinitely (set in the Sound Property Editor).
Back in the sequence containers, set each container's playlist so that the gear switching sfx plays and then the looping silence plays.
Back in the switch container, assign the sequence container objects to the correct switches in the assigned objects pane.
Now add an event action to the event that starts the car engine so that it also starts the switch container we just created. When the RTPC gets updated, the RTPC will set the proper switch, play the correct sound, then start looping silence until it gets another set switch call from the RTPC.
I tested all this in the authoring tool just now, so I'm sure it works. Let me know if you have any trouble.