The latest update on my OnePlus 5T (to Android P) make OnePlus Gallery the app that opens when you click on the gallery/image icon at the lower right corner.
You have actually disable the Gallery app.
To do this, go to the phone's settings, and the applications menu there.
Find the Gallery app, and click on the Disable button.
From then on, the camera will open the default app for photos, which you should make sure to set to Google Photos.
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