The 31-year-old's visit to Paris was a surprise to Mendes, who is currently filming her new movie Holly Motors in couuntry.
"They were talking all the time so she had no hints that he was coming. She was really homesick and missed him, even though she practically just left (America) to shoot," dailystar.co.uk quoted a source as saying.
Gosling became romantically linked with Mendes September this year, following her split with longtime boyfriend producer George Gargurevich.
Although the actors have yet to publicly confirm their relationship, they have been photographed together on numerous occasions.