I did some work recently on my vehicle's engine,I used the opportunity to inform the mechanic to service the gear,since then my gear has been leaking having bought oil seal, please guys advise, should I still buy another oil seal or look for another mechanic?.what could be the cause of the leakage?

You didn't specify the kind of gear automatic or manual?
Mind you we have gear box specialists, is not every mechanic that can handle gear box repair.

I will advise you make enquiries on how to get a good gear box technician that will solve your problem.