[PRW]

I'm pretty sure it's every 3 years or every 60,000 km, whichever comes first.

 

[PRW]

And 🤷‍♂️

 

[PRW]

The oil in the gearbox must be changed every 60,000 km or every 3 years, whichever comes first.

 

[PRW]

I just called up mercedes instead, told them my reg for the w177 and the advisor looked on their system and told me the gearbox service for A200 (my reg) is due every 77,500 miles and was about £220.

That doesn't sound right to my ears, but maybe your model doesn't require it before 🤷‍♂️
Personally, I would have it changed every 3 years, it's not particularly expensive and helps to provide a smoother gear change and maintain the gearbox.