A GDI "ROM" is rarely just a single file. Unlike an ISO, which is one large file containing everything, a GDI set is typically a folder containing multiple files.
That is an interestingly short review. Since you didn’t provide additional context, here are a few ways to interpret as a review: dreamcast roms gdi
Do not use CDI unless you intend to burn a disc to play on a real Dreamcast console. For PC emulation, you should only use GDI or its compressed cousin CHD (which we will discuss next). A GDI "ROM" is rarely just a single file