Avengers: Infinity War shows the two together romantically for the first time and while it’s unclear how long their relationship has been like this, there have been years that have passed between the events of Guardians Vol. 2 and Infinity War from a story standpoint, Peter and Gamora certainly see each other as family even if it’s only been recently they’ve become more than that.
Peter Quill’s relationship with Yondu (Michael Rooker) was a difficult one and while the end of Guardians Vol. 2 saw Quill saying goodbye to a father figure, it was more about coming to terms with his past.