The only GMT movement that ETA produce is the 2893. Enterprising folks in far off countries have modified 2836 movements for quite some time with a homebrew GMT complication added to the movement.
You can also do a more complicated swap by taking the balance wheel, pallet fork and escape wheel and replacing them on the 2836 movement to gain a low beat movement.
Here you can see the original 2836 movement on the left. I have removed and replaced the following components with the original swiss parts on the 2846. These are parts that are modified by the chinese factories to allow the GMT function.
Part 2595 – Date Jumper Maintaining plate – This part has been modified slightly to allow for the modified Intermediate Date Wheel
Part 2543 – Intermediate date wheel – This has a small secondary gearing placed atop the original gear. This turns the GMT wheel during the normal cycle of time. It meshes with a gear on the underside of the GMT wheel.
Part 242 – Canon Pinion with Driver – Taller canon pinion to account for the additional GMT wheel.
Part 255 – Hour Wheel – as above, taller to allow for new GMT wheel.
Part 2567 – Day Corrector – originally the part to allow quick setting of the day disc. Replaced with a new bi-directional gear that allows quick setting of the GMT hand.
Seen below is the GMT wheel itself. The upper gearing meshes with the gear running where the day corrector normally would and the gearing on the underside runs off the intermediate date wheel. And finally as installed on the finished movement with the brass washer in place.
