It said HM Revenue & Customs had expanded its probe into suppliers GVC used in Turkey to include other entities within its group.
GVC sold its Turkish business in December 2017.
The company said it was cooperating with the investigation.
GVC was informed of the development on Monday but said it had yet to be informed of which of its subsidiaries were being investigated.
It said it was "surprised by the decision to extend the investigation in this way and are disappointed by the lack of clarity provided by HMRC as to the scope of its investigation".
"HMRC has not yet provided details of the nature of the historic conduct it is investigating, with the exception of a reference to section 7 Bribery Act 2010, nor has it clarified which part of the VGC group is under investigation."
Section 7 states that a commercial organization is guilty of an offense if a person associated with that company bribes another person to obtain or retain business or to obtain or retain an advantage in the conduct of business.
GVC’s share price was down 10.6% to 778.2p on Tuesday following the news.
The UK Gambling Commission told the BBC that it was "assisting" with the investigation.
HMRC declined to comment.
In July 2019, VGC was forced to deny reports that it was still benefiting from its former Turkish subsidiary, stressing that it no longer had ties with the business.
“The board reiterates the fact that subsequent to the disposal of the group's Turkish-facing business, VGC has no activity either directly or indirectly linked to the Turkish market,” GVC said.
“Furthermore, the board also categorically refutes suggestions that the group, or senior management, continue to benefit from any operations servicing the Turkish market."
Liz Coleman, the former HMRC tax inspector, and consultant of advisory firm Integrated Dispute Resolution, said the investigation could go on for some time.
"HMRC investigations are usually lengthy and considering the size of VGC it could be going on for years, although if there's a targeted approach it could be shorter," she said.
"They have to analyze all the computer records too, so this could add even more time."
The HMRC's decision comes just a week after Kenneth Alexander announced that he was stepping down as GVC's chief executive.
Mr. Alexander took over in 2007 and oversaw the company's acquisitions of rivals Ladbrokes Coral and Bwin.party.