Kathryn Mayorga alleges she was raped by the footballer in a Las Vegas hotel room in June 2009. Ronaldo has strongly denied her claim.
Gemma dated Ronaldo briefly in 2007, and she could be questioned under oath about her relationship with him.
There is nothing to suggest that he assaulted her.
Mayorgas lead attorney Leslie Stovall says Ronaldos exes will have a great insight into his conduct to help build a picture of Mr Ronaldos behaviour which could prove vital to our case.
I want to speak to any girlfriend who knew or knows him intimately, she told The Mirror.
The Portuguese footballers other past loves include model Irina Shayk, who he dated from 2010 to 2015.
Ronaldo, 33, vehemently denied Mayorgas allegation in a statement.
He tweeted: I firmly deny the accusations being issued against me. Rape is an abominable crime that goes against everything that I am and believe in.
Keen as I may be to clear my name, I refuse to feed the media spectacle created by people seeking to promote themselves at my expense.
My clear conscious will thereby allow me to await with tranquility the results of any and all investigations. (sic)
Georgina Rodriguez, his girlfriend and the mother of his youngest daughter, retweeted his statement and added a prayer hand emoji and love hearts.
Mayorga alleges in her lawsuit that she was pressured into signing a nondisclosure agreement to keep the alleged rape secret and is seeking no less than $200,000 in penalties and damages.
Gemmas publicist has been contacted for comment.