If you are experiencing this problem on GCam 7.3, there are two fixes:

1. Check the settings menu. Some versions allow you to "use a third-party gallery" (the default gallery app on your phone).

2. Install the Google Photos app (default gallery used by Google on their phones). There's no need to log in and backup your pictures/videos.

You need to wait until HDR processing is finished in order to see the picture. This may be annoying, specially if you have slow/old phones.

This is the result of changes made by Google to version 7.3.x. If you don't like the new behaviour, use GCam 7.2x or older.

If the modded 7.3 GCam you're using supports 3rd party gallery apps and it still doesn't work as well as before, try using the Google Photos Go app (stripped-down version of Google Photos, without file upload). Some default gallery apps don't work well with GCam. Using Photos Go should fix most issues and it also handles the HDR processing delay better.

1. Install Google Photos Go
2. Enable the "use a third-party gallery" setting (or similar, not all versions use the same name)
3. When viewing a picture from GCam, select Photos Go

- Play Store: https://play.google.com/store/apps/details?id=com.google.android.apps.photosgo
- APK Mirror: https://www.apkmirror.com/apk/google-inc/gallery-go-by-google-photos/

Thanks to Ujjwal for the suggestion.