It's taken me a while to catch up with this design! So many of you pointed out that you pull the handle out to operate the gear lever and not the trigger. So a quick rethink of the design and by changing one hole to a slot, both the trigger and pulling out the handle will now operate the gear lever. If you have already created the gear lever using the old design. You only need to reprint the main tube or create a slot in your older version as shown below.
The updated design can be found at https://www.thingiverse.com/thing:3562570