If oil is leaking and covering the engine, whether partially or completely, it's standard practice to clean the old oil off the engine to try and trace the leak. If you don't clean the oil off, how will you be able to see where it is coming from?
What work needs doing, and how much it will cost, will depend on the diagnosis when the cause of the leak has been determined.
Having said that, if the leak worsened after the garage replaced a gasket, you might be better off finding a different garage.
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