The Nevada State Athletic Commission confirmed Siver's positive test to ESPN.com on Saturday.
NSAC chairman Francisco Aguilar said a complaint is currently being prepared by the Nevada Attorney General's office.
The substance hCG is linked to anabolic steroid use and/or weight cutting. The NSAC tested all 22 fighters at UFC 168 and all other tests came back negative.
Siver does have the right to an appeal.
However, he faces a likely fine and suspension based on the current result. His unanimous decision win over Gamburyan would also change to a no-contest.
Fighting out of Mannheim, Germany, Siver (22-9) is 3-1 since dropping to the UFC's featherweight division in 2012. Prior to that, he had fought the majority of his UFC career as a lightweight.