Another thing that bugged me about the Lego GT3RS trans was that you could pull the upshift paddle forever and it would just cycle through the gears.
This is because the shifting is done off of an axle with these little “cams” for lack of a better term. From the instructions, there is no way to stop the rotation at 4th gear, allowing you to go from 4th to 1st and then to 4th again with just a pull on the downshift paddle.
Don’t worry because I’ve done something that Lego should have done in the first place.
Put that little bit that you’ve made in there and you’re good to go.
So I hope this helps.