If the gear system in my car stopped working, should I replace it and sell my car or keep it? How reliable will the spare parts be?

HI, It will all depened on the kind of repair your going to do with your car if you are going to replace the whole gear assembly I would recommend that you still keep your car for may be around 5years depending on how you would be using it. But if your going for a fast repair or changing only the gear that is already damaged I would recommend for you to sell it for replacing only part of the gear is not that reliable and would become a very expensive repair in the long run.

Is it manual or automatic? HOw much are the repair quotes? HOw much is the car even worth? Is it old car - are you thinking about replacing it in short term or going to keep it for long time? Are you getting official dealership with original parts or some independent repair shop?  Answer all those questions and just compare costs.

there will be many reason for jamming the gear and most of the reasons can be sorted out without spending much money,you can sell if the repair cost higher amount than your normal maintenance cost

regarding gear issue we need to check the first inspection of gear cable, gear lever  then it is minor replace the part & you can use it.  If at all there is gear box problem we have to dismantle gear box find any gear level teeth damage our it need to be overhual gear box. If any serious issue are found replace the gear box assembly. You can use it. If ageing of the car is more than 5 or 7 years then you can think of disposal.  

first check if the gear lever connection to the gearbox is still intact. if not try replacing the connection cable/link. if that does not solve it then the gear box needs to be dismantled and checked for error. depending on weather the problem is with the gears or with the synchronizer and the availability of the parts we can tel if selling is better or resolving the problem is better 

Need overhaul gearbox and replace fault part with new one.This is no big problem and you should not sell car, of course if problem with your car only this one. 

  replace it with new gear system and keep your car if you sure that their is no more problem will face you later