Gold Farm - 91 is fully-owned by private individual Ivan Angelov, who is also a major shareholder in Gradus, the company said in a bourse filing.
The purpose of the acquisition is to optimise certain administrative and operational expenses, Gradus said.
Ivan Angelov held a 20.36% stake in Gradus at the end of September, according to the company's interim financial statement.
As at 13:15 CET on Wednesday, Gradus shares traded 0.90% lower at 1.655 levs on the Bulgarian Stock Exchange.
(1 euro = 1.95583 levs)