Unfortunately you can only change I to Google gallery. I have the same issue, the sharing function of Google photos and Google gallery sucks. They have their own quick sharing option that is much worse then built in Android one. You have to click more… to open the default one. Please give us option to disable Google photos share or force default Android share. Or at least make option to pick third party gallery as default
Option to change default gallery app
Exactly my point being this only
Try disabling the Google photo applications, it will take you by default to the app you install.
2 months later
RuudHesp Disabling google photos doesnt do anything, once you take a pic and click the photo, it doesnt open any gallery
Juan_Blade43v3r_Rambo Uninstall the Google Photos app. It takes me to the only photo gallery app I have installed.
6 days later
No that’s not happening, even though google photo app is uninstalled, the other galley app wont open as a default. I should open the app from the Home Screen to open but I can’t access it directly from the camera as a default app. So now that an issue to be solved in the next update.
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It works on my phone. A third-party app opens for me.
a month later
Pls provide default gallery app for nothing. Also option to restrict gallery viewing from camera. And selection of gallery in default apps.
9 months later
How can we set nothing Gallery app as our default gallery rather than Google photos !?
SUMITHINDOLIYA This is currently not possible and might come in a future update.
Mr Steel ohkayy
6 days later
How to set Nothing Gallery as default, when I disable google photos and take a picture I cannot access from the camera to the gallery (Nothing 2a, OS 3.0, Nothing Gallery).
Hey mate, not possible at the moment but let Nothing know you want it via settings > system > feedback 
20 days later
How can I set Nothing Gallery as default. When I capture a photo it shows in Google Photos when I tap the preview. The gallery was released so that the camera processing will be faster, but I don’t see any option to set it as default.
ShiveshChauhan hey mate. You can’t do this at the moment. This might come soon and I’d log it as feedback via settings > system > feedback so the Nothing team can see it! Cheers.
10 days later
ChronoSee This is not a problem. The camera’s AI optimizes the best performance for your photos when you take them, so it may take a while to save the photo.
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I recently started using the newly launched Gallery app of Nothing. But, when I am using the Camera app, it wont open photos in the new Gallery app, instead it tries to open Photos app from Google. I like the Gallery app very much but there seems to be no way to change default app for photos in Nothing OS 3.0. Please could you guys fix this small issue?
Merged 9 posts from Default gallery app., Nothing Gallery as default, Nothing Gallery as default, Setting Nothing Gallery as default , and Need of an important feature in Camera app.