On my google activity page, it is showing Used Gallery followed by a Used Camera a few minutes later. Not sure why it is saying Used Gallery when I didn't access any photos from there.
Your Google activity is your app activity. Have you tried opening your camera to see if the two always show up together? Did you take pictures at that time, you only specify that you didn't open the gallery? I don't think it monitors background app usage so I wouldn't imagine that it recorded something being backed up but maybe your camera triggered it at the same time...
Welcome to Android Central! Which phone do you have? If your stock Gallery app can sync with your Google Photos in the cloud, it might be periodically syncing, thus showing up as a Google activity.