Hi Andris,
I recently bought and installed a Healtech Speedo healer and digi gear indicator...
The gear indicator has to be plugged in line with the speed sensor, and then has to be soldered in to one of the wires going in to the ECU. If you look on the healtech website they have installation guide downloads, I'd post a pic of the instructions that came with mine but I'm at work at the moment...
https://www.healtech-electronics.com/products/gpx/ This is the one I have, it's the GPX-H01 variant for our bike, if you look down the bottom of the page it has a section for "Install Guides".
Cheers,
Jon.