There's simply no way Detective Pikachu could feature every single Pokemon and do them justice. Some of them had to be left on the cutting room floor. According to a Comicbook.com interview with co-writer Dan Hernandez, both Golbat and Gardevoir were slated for an appearance, but never made it to the final cut.
"For a while, we had a cool sequence with a Golbat that I really liked. That was a really cool sequence that just didn't quite make it to the end of the process. ...Gardevoir had a cool moment. There were so many different, all the different drafts along the way."
Unfortunately, we don't get any details on what was planned for either Pokemon. We'll just have to imagine scenarios of our own.