Cory Gardner (R-Colo.) has not made any substantive comments regarding the 2020 U.S. presidential election outcome.
According to the Washington Post, only six Republican senators (out of 53), seven Republican governors (out of 26), and 10 Republican members of the House (out of 197) have thus far acknowledged that Biden won. On Nov 17, 2020, Senate Cory Gardner was asked whether he considers Biden, the president-elect. He replied he was not going to play their gotcha questions. It's an easy question if anyone has any integrity, he said.
Gardner has not made a public statement accepting Biden's victory, referring to him as the president-elect, or congratulating him or Vice President-elect Kamala Harris. So, the claim is unverifiable at the current time.