You would not get a error message. Photo Gallery just open but not display the files. Microsoft's explanation is almost make some sense (not really, if you read it again and think about it for a second). [Windows XP's Photo and Fax Viewer it replaced does support GIF. see windowshelp.microsoft.com/Windows/en-US/help/490793a6-70f0-453b-80d6-0b1a0e9b1c401033.mspx]
You’ll rarely need to use a file type other than JPEG or TIFF. The
bitmap format (.bmp) is an older standard that creates needlessly large
files. This wastes disk space and makes it difficult to send these
pictures in e‑mail. GIF and PNG are commonly used on webpages, but
you’ll find that JPEG works just as well.
Why can't I see some of my pictures in Photo Gallery, even though I know
they're in my Pictures folder?
Photo Gallery is intended to show and edit digital pictures from cameras
and scanners, so some older file types that are not commonly used for
digital pictures today will not appear in Photo Gallery. If you have
picture files with .gif extensions, for example, they will not appear in
Photo Gallery and can't be edited using the Fix pane. If you want to see
these pictures in Photo Gallery, you should change their file type to a
format that Photo Gallery can display (JPEG is recommended).