How to Upload Google Photos to a Facebook Page From Within the Android Facebook App? [Solved]
The Facebook and Messenger apps only look for photos that are stores locally on your phone. The photo Google Photos aren't "really" stored on your phone (only small, low-res thumbnails are stored). So you have to download the photos from Google photos from the cloud to your phone (select the photo in Google Photos, tap the symbol on the upper right and then select download) before Facebook can "see" them.
You cant, However you can do it using the google photos app by selecting the pic and then the share option and selecting facebook.