The 31-year-old actress received a government grant to study at the Royal Academy of Dramatic Art (RADA) in London in her youth, but Gemma never dreamed of being a star of the silver screen and was instead focused on making it on the stage.
She said: "I never thought I'd work in film. I always thought I'd be just a jobbing actor in the theatre.
"I remember in my third year at RADA, I did a budget - about the only one I've done - calculating how much it would cost to live in London. You are told you might work once a year. That's the life."
Gemma - who made her film debut in 'St Trinian's' in 2007 - admits she doesn't know what she would have done if her acting career hadn't worked out.
She told the Irish Times newspaper: "I'm a coaster. If it hadn't worked I'd have found something else to do. It's so precarious. Things fall apart. It's mad. It's not a job for anybody who wants to plan."
Meanwhile, Gemma recently admitted she has been offered different types of roles ever since she started to voice her dislike of big-budget Hollywood movies.
She explained: "I'm approached with projects that are much more female-centric because I've spoken out about it so much, which is a good thing.
"But I don't necessarily look for these kind of feminist messages or anything like that, but it's always quite nice when there's a message in there.
"The future projects I have are all kind-of about women that are great, or not great, they're idiotic, or silly, they're just about women so that's a good thing."