Happened to me out on a group training run five years ago. She and I are now engaged and will be getting married in July.

When I still had a personal Reddit account, I would be on the dating and relationship subs and promote the idea to do something every week where you see the same people. even better do two or three such things every week. That's what I did, and I quickly went from zero local friends to dozens.

The gym is a fine place to do that but only if you're doing classes where there's an expectation that people will be socializing. I made some of my best friends in such gym classes including my current best friend. She indirectly introduced me to my fiancé because she suggested I join a running club to train.