"Indeed, he has every right to be proud of (the purchasing card program's) success and national recognition, and the public benefits by this successful program," the opinion stated.
"Here, however, in the course of promoting his office's electronic payment program, he lends the prestige of his public position to a private business for which there is no overriding public benefit."
The commission said that even though Gainer did not get paid for appearing in the videos, he still might have violated the Ethics Act's prohibition against using public office for private gain by appearing in the video during a contested election campaign.
Gainer defeated Republican Larry Faircloth, an Eastern Panhandle resident, by a 57 to 43 percent margin in November's general election.
Since the video was online during the campaign, the commission said it offered Gainer a potential advantage.
"Although it may not be possible to quantify the benefit that may have inured him as a result of this publicity, it is an intrinsic benefit that his opponent did not have access to during the campaign," the opinion said.
But since advisory opinions from the Ethics Commission are only concerned with conduct that may occur in the future, the commission said it "is unable to opine as to whether (Gainer) used his public office for private gain."
In a brief statement, Auditor's office spokesman Justin Southern said Gainer intends to abide by the commission's decision going forward.
"We respect the Ethics Commission's opinion, and will follow its advice," Southern said.
Contact writer Jared Hunt at busin...@dailymail.com or 304-348-4836.