any good servicing workshop to recommd

Around Sitra / Saar / Isa Town, there are plenty of car repair shops that could potentially service your car much cheaper than your car manufacturer's workshop.
I cannot recommend any specific repair shop as I rent my car and if anything goes wrong with it, I would take it back to the rental place.

Good luck! Keep us posted if you find any.

I have owned numerous garages in the past and people have brought their cars to us for a second opinion after main dealer has said clutch or gearbox gone £600 thank you. We have found that a suspension arm or something small has gone but the company know the customer is not a mechanic and try to make as much money as possible. My neighbours brother is a mechanic and virtually rebuilt our car for 120BD (accident damage) this was the front and rear ends of the car rebuilt with new parts and labour. I would ask that you get a 2nd opinion from another garage. There are a lot just past the camel farm in Janabiyah. If you want the details of our mechanic friend then PM me.

A car with 37,000 mileage and the gearbox needs replacing? Hmmm, i'd probably get a second opinion. If you do go to another garage, just ask them what needs doing on the car - I wouldn't tell them about being charged this amount for the gear box. Would be interesting to hear what they say and what cost they put forward.

Do you have the previous service history of this car? Also i take it this is an automatic transmission? I can only think a new gear box may be needed if the previous owner was a really bad driver or the car was never regularly serviced.
I have heard for automatics, the oil needs changing every 5,000km although i cant be sure as i'm no expert in this field of engineering.

Garages here are known for adding unnecessary charges on parts that require replacing or need fixing. It happens in most places. I ve heard stories of BMW & Toyota and they are not far from what you mention here.

I hope others (who know this field well) on the forum, will be able to advise you or recommend a decent mechanic or garage who can give you a second opinion.
Jo has even offered you details of their mechanic, so you could take up this offer too...you have nothing to loose..

Good luck and keep us posted..

Jaz