Unlike Gemini Classic, Gemini 2 filters out not only identical files but also those files that slightly differ from each other. They are called "similar files."
Gemini analyzes items within the specified location by comparing the exact lengths of the file data and a set of other file parameters. When the app detects absolutely identical files, it puts them to the All Duplicates category and offers to remove these files with Smart Cleanup.
However, if files slightly differ from each other, Gemini puts them to the All Similars category that requires your review and further manual selection for removal. For example, Gemini considers "similar" two photos made with a 2-second delay where an object and background are the same. This way, you can easily free up much more space on your Mac while keeping all unique files.
Note that similar files aren’t selected for removal by default, but Gemini 2 may offer you to create a new Smart Selection rule that automatically adds these files to the cleanup list.