That represent 23.5% stake of the company.
SoftBank currently holds 25.8% of GungHo shares.
Priced at ¥294, the deal would be worth $685 million
The move follows years of acquisitions from SoftBank, which have left it with record debts of over $110 billion.
Some of its deals - such as GungHo, Supercell and Alibaba - have been positive, but the 2013 acquisition of US carrier Sprint - a $13.6 billion deal - has pulled down SoftBank shares ever since.
Indeed, SoftBank recently split in two so its better performing companies weren't being held back by Sprint's performance.
As well as GungHo, SoftBank is planning to sell $10 billion in Alibaba shares, and if rumours are to be believed, its 73.2% stake in Supercell.