Short answer: .giz files aren't going to help you.
Long answer: What you saw us doing in the forums was using a Sega Genesis/Mega Drive emulator called "Gens Plus!" which allowed us to record savestates and input strings for playback, which results in "videos" of ridiculously smaller file size (Since the playback is merely the savestate reloaded and the inputs reissued from there). That's obsolete now, since most current-era computers can handle screen capture.
To my knowledge, that only works with Genesis/MD and Sega Master System/ GameGear games, because that's all Gens Plus! supported.