Google Photos gallery app on Android devices is a crawler, it searches for images everywhere on your device it has access to and shows them in your gallery. If games do contain unpacked unobfuscated images, they too will appear in your gallery.
As far as I can see, Google Photos does not have any options to exclude some folders from being scanned for images. Some other gallery apps do.
Your 3 options are either not to play games on your device or not to use Google Photos as your gallery app. Or not to care about the images showing up in your Google Photos gallery.