The Sega Dreamcast used proprietary optical discs called GD-ROMs, which held roughly 1 GB of data. A GDI file is an exact, 1:1 sector-by-sector rip of these original discs.
If your file does not come from these lineages or a verified TOSEC (The Old School Emulation Center) .DAT file, you are not dealing with a "better" collection.
