The download is compressed. Use 7-Zip (Windows) or The Unarchiver (Mac) to extract the .img file from the .gz archive.
Why bother downloading the latest "top" build? Here is what you get with Batocera 38/39: batocera iso download top
Batocera on Raspberry Pi is legendary. The top build for Pi currently focuses on Vulkan drivers for the Pi 5. The download is compressed