Cameroon had had chances to score and within eight minutes as Achille Emana had his shot across goal tapped away by goalkeeper Didier Ovono and onto the post.
After Cousin, who plays for Hull City in England, scored Cameroon pushed to gain at least a point.
Geremi, the former Real Madrid and Chelsea midfielder, now at Newcastle United, had a 25-yeard strike saved in the first half and a powerful free kick tipped onto the bar by Ovono two minutes before full time.
Meanwhile, Samuel Eto'o, Cameroon's Inter Milan forward failed to pose a threat to Gabon, who were making their first appearance in the tournament for 10 years, throughout the match.
Cameroon, who are the fourth World Cup qualifying team not to win their opening match in this years African Cup of Nations, almost lost by 2-0 as Gabon's Willy Aubameyang smacked a free kick from 30 yards onto the bar, with the ball bouncing back to the ground in front of the goal line.
Gabon were allowed to take top position in Group D after the later kick-off between Tunisia and Zambia, the other two teams in the group, ended 1-1.
Outsiders Zambia took an early lead against the North Africans with forward Jacob Mulenga, who plays for Dutch side FC Utrecht, putting them ahead in the 18th minute with a neatly struck shot.
Zambia dominated the game but Tunisia, the 2004 champion, scored against the run of play five minutes before half time.
Striker Zouheir Dhaouadi finished a smooth passing move from near the penalty spot after a cut back pass from Youssef Msakni left him with a clear shot at goal.
Both sides showed spirit in the second half and Zambia could have gone ahead again but an unmarked Rainford Kabala missed his header from close range with 20 minutes to go.