The top two in Group F meet in Athens with a point between them as Croatia coach Slaven Bilic's leaders look for a win to reach the finals while the hosts must avoid defeat to stay in the hunt for first place before the last group games on October 11.
"When the draw was made we said that the two games with Croatia would be the most important and so it has proved," Greece coach Fernando Santos told reporters.
"After the 1-1 draw in Croatia we knew it was a very good result because it meant we kept our destiny in our own hands."
Gekas publicly apologised for what he called an "error of judgment" in a statement he issued on his return to the squad.
"The match against Croatia is like a cup final for us and we all believe we will win it," defender Socrates Papastathopoulos said. "I think we'll be in the Euro 2012 finals automatically."
Neither team can finish below second place in the group, which at worst would lead to a spot in the qualifying playoffs for next year's finals in Poland and Ukraine.
"Defeat would not be the end of the world. Whoever happens to lose this game will still end up second and will get another chance," Bilic said.
There were no major changes to Croatia's squad from their last game against Israel on Sept. 6 with the only surprise being Bilic's decision not to call up forward Ivica Olic who was deemed unfit following a hip injury sustained in August.
Striker Eduardo skipped Monday's training session due to a minor injury and Vedran Corluka had a slight ankle problem but both are expected to have recovered by Friday.
Croatia's final qualifier is at home to Latvia next Tuesday, while Greece travel to Georgia.
