Mainz remains second behind Dortmund, which is now assured of top spot going into the winter break. The leader has a seven-point cushion and a game in hand ahead of Sunday's match at Nuremberg.
Wolfsburg and Werder Bremen continued their disappointing seasons by playing out a scoreless draw with both teams missing a penalty in the space of three minutes.
Hannover completed a comeback over 10-man Borussia Moenchengladbach with two goals in three minutes from Mike Hanke and Didier Ya Konan.
Stuttgart and 10-man Hoffenheim shared the points in a 1-1 draw. Isaac Vorsah had been sent off late in the first half.
Freiburg beat Hamburger SV 1-0 with a goal after three minutes from Papiss Demba Cisse.
In Frankfurt, Marco Russ had given Frankfurt the lead with a volley from 15 meters (yards) before Andre Schuerrle equalized late in the first half.
Gekas secured the victory in the 84th with a perfectly taken penalty.
Torsten Frings missed the chance to put Werder in front when his penalty was deflected onto the post by Wolfsburg goalkeeper Diego Benaglio in the 74th. Edin Dzeko repaid the favor three minutes later when he ballooned his penalty kick high over the bar to complete a frustrating afternoon for both sides.
A red card for Raul Bobadilla for stamping proved costly for Borussia Moenchengladbach. His side had been leading at halftime through a free kick from Michael Bradley. It was Moenchengladbach's first shot on target but the American fired home from 40 meters (yards).
Hannover fought back with quick-fire goals from Hanke and Ya Konan and the win brings Hannover to third in the table, pending Leverkusen's game at home to Cologne on Sunday.
Alarm bells are ringing for Moenchengladbach as it remains rooted to the bottom of the table.
Hoffenheim held out for a draw at Stuttgart despite playing for 55 minutes with 10 players after Vorsah was dismissed. Sejad Salihovic had given the visitors the lead but Stuttgart's Martin Harnik scored a well-deserved equalizer following a precize cross from Cacau after 34 minutes.
Stuttgart was unable to capitalize on the extra player in the second half, however, and remains in relegation quagmire with this draw coming after two successive defeats.
