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.
Unfortunately, this question cannot be answered right now.
When Gon was younger, Mito told him that both of his parents were dead. However, it is later found out that it was a lie, because his father Ging was obviously alive and well.
Before the Greed Island Arc, Gon got a tape from Ging. At the end of the tape, when Ging was going to tell him about his mother, instead of listening to it to the end, Gon just stopped the tape and stated that Mito was his mother.
There is a lot of speculation as to whom his mother could be, but until Togashi supplies the answer, we will never know.