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.
Just check you have enough space on your Google drive as I believe it's no longer unlimited storage for photos like it used to be so any photos and backups etc may be included in your free 15gb e.g if you have other files on your Google drive It could easily fill up