It is not Mito obviously.
When Gon is listening to the tape that Ging leave him, Gon stop the taped when Ging ask if Gon want's to know about his mother. I've waited so far but until the last episode of the anime it is not yet revealed.
If Gon just listen to the tape, then the mistery should already solved. Is there any answer that can be found in the Manga? I'm prepared for spoilers.
If Leorio was mad about how Ging acted when he knows the condition of Gon, I'm sure Leorio will be more mad if he knows that there's a mother to Gon.
If she is ever alive, then who is she.