Merceron, rated France's best kicker, missed two penalties and a conversion as Les Bleus slumped to a 25-17 defeat against England in their RBS 6 Nations opener on Saturday.
France coach Bernard Laporte will name his team on Wednesday for their next match against Scotland on Sunday and could opt for Gelez, the 24-year-old Agen fly-half.
"I am ready for it," said Gelez, who earned his first cap in France's 30-10 win over South Africa in Paris last autumn.
Gelez is optimistic, but insisted he did not care how long he had to wait to become a "first-team member" for Les Bleus.
"Confidence does not come after two matches," he said. "It took me 18 months just to get used to professional rugby with Agen and two years to become a member of the first team.
"(Jonny) Wilkinson and (Australia's Steve) Larkham are branded the best fly-halves in the world, but Larkham needed four years to impose himself as a first-team regular.
"As far as I am concerned I don't want to make too many calculations, wondering whether I will start or remain the number two in France.
"If I am given the chance I will seize it. My international career must kick off at some point."
Gelez believes France need to to inject more creativity and unpredictability into their play - qualities he thought were in short supply at Twickenham.
"We started the game with a fixed tactical scheme," he said. "But by now our rivals can read our game better and understand the way we think.
"Last year's surprise factor is over. We must now show we have other weapons to break our rivals' defence.
"We must be more autonomous and creative while adapting to (unexpected) situations better."