If you bought it from a garage/trader then I would suggest that you take it back to them and reject it and get your money back. They are unlikely going to want to repair it as that will probably cost more than the car is worth. Sadly if you bought it privately then I think you have discovered why the owner was getting rid of it and unless you can prove that they have misled you regarding the fault with the gearbox issue then you will either have to pay to have it fixed or cut your losses and offload it for whatever you can get for it.