Cheap 'n' Cheerful
Just be wary that the cheaper (mostly chinese) gear indicators on ebay are mechanically actuated by the gear lever and are a bit "Heath Robinson", partuclarly in the way the mechanism works.

The Middle Ground
The Acumen ones are reasonably priced, but are pretty laggy, slow to react and can be easy confused.

At the Sharp End
The best are the Glpro X-Type. Fast reacting, easy to setup and unlike most you get a bike specific wiring kit (GPX-Y01 for the XT660) with the right plug on it for the speedo-ECU sensor connector. Not the cheapest though.

In a nutshell, you pay for what you get with gear indicators...

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