To stop synching photos to OneDrive for Samsung Gallery, first open Samsung Gallery. In the bottom right of the app, go to the menu (three horizontal lines). Then go to settings. The first option lets you disable synching to OneDrive.
Next, to enable photo synch for Google Photos, open Google Photos app. In the top right, click your profile icon. There is then an option Photos Settings. Go to the option for Backup. Within this is the option to enable Backup to your Google account.
Hope this helps.