Gocycle G2 and Gocycle G3
The Gocycle G2 and G3 have electronic shifting, which is actuated by a servo unit inside the Cleandrive®. Shifting is calibrated during production but it can be re-calibrated manually. See Section 7.6 of the Owner's Manual for a full description of this process.
Gocycle G1, Gocycle GS and Gocycle GX
The G1, GS and GX models have mechanical shifting, and use a Jagwire CM157BJ Rocket Adjuster to adjust the shifting.
The rocket adjuster is two halves and is spring-loaded. To increase tension on the cable, hold still the smaller half with 'Jagwire' printed on, and rotate the other half anti-clockwise.
The feeling in the shifter when you change up should feel positive, but not overly stiff. If you find you need to really wrench the shifter round to change up the gear, it is too tight.
You can tell if the gear is not adjusted correctly, as the gears may slip whilst pedalling. i.e. it feels temporarily that you have zero pedalling resistance. This may happen on a new bike as the gears bed in as the shift outer casing compresses. You would need to increase the tension on the cable in this situation.
There is a limit to the adjustment of the rocket adjuster. If you unscrew it too far the larger half will just spin, as the thread is not engaged. If this happens on your Gocycle, a new cable will likely be required.
You can find more info in Section 6.6 of the GS Owner's Manual.