Tzolis’ low left-footed strike from Onyedika’s cross put the home fans in a jubilant mood and left the tie in the balance.
The Norwegian side took the lead in the 20th minute when Hogh’s backheel set up Sondre Brunstad Fet, who calmly slotted into the bottom corner.
Leverkusen will look to seal a round-of-16 spot in the return leg in Germany next week.
It was Gordon’s 10th goal in this season’s Champions League, more than Harry Kane (8) and Erling Haaland (7), with only Kylian Mbappe (13) ahead of him.
The first leg of the playoff ties will be played on February 17 and 18 while the return legs will take place on February 25 and 26.
Liverpool returned to winning ways in the Champions League on Wednesday night, beating Marseille 3-0 in France in their penultimate match of the first round.
The final whistle confirmed Arsenal’s dominance as they remain top of the Premier League standings.
Freiburg opened the scoring in just 12 minutes through Japanese international Yuito Suzuki, who finished a perfect close-range effort.
Barça took the lead inside the opening minutes through a powerful solo effort from Robert Lewandowski into the bottom corner beyond Unai Simón.
The club has requested the written justification of the ruling from UEFA to appeal the decision.
