The answer depends on which version of Gwen Stacy you're interested in:
In Spider-Man: Across the Spider-Verse: Gwen Stacy is likely between 16 and 17 years old. This movie takes place a year and four months after the first movie, where Miles Morales was 14 and Gwen was a bit older.
In the comics: The age difference between Gwen and Miles can vary. In some versions, there's a bigger gap, with Gwen being around 19 while Miles is nearly 17. However, the movies show a smaller age difference