It comes after a lengthy regulatory approval process, with the Japanese investor originally touting a much larger figure of "up to $150 million".
The final investment buys Softbank a 7.5% stake in GBM via its subsidiaries, according to the securities filing.
The new funds will allow the more than 35-year-old company to "continue its investment strategy in technology, human capital and marketing," it added.
($1 = 20.0144 Mexican pesos)
(Reporting by Carolina Pulice, Editing by Isabel Woodford and Cynthia Osterman)