My head gasket was fixed but I still have a leakage

Presumably the timing belt would have been removed in order to facilitate the head gasket change. It very much depends on the type of engine you have but I have frequently seen water pump failing to reseal after they've been removed and refitted.

Personally, I'd have changed the water pump (or at least the o-ring / seal / gasket) and the timing belt as part of the task of replacing a head gasket. If definitely makes sense that the coolant could be disappearing thanks to a faulty water pump / water pump seal.

If you can give me specifics of the make,m model, year and engine type in your car I might be able to be more specific.

There is a possibility your mechanic did repair the wrong thing if there was no coolant in the oil (the oil turns grey/white if it gets mixed with water). If it was not, the head gasket might have been ok. You mention some leaking from the radiator. Have you replaced the radiator cap? Sometimes they leak. The leak might be in the radiator itself. If not, the water pump probably needs to be resealed or replaced. If these suggestions don't fix it, you might get rid of a lot of cash as in some serious repair might be needed.