David,
I have not tried this but it is worth a shot, I had to bring in transparent symbols from autocad.
1. open the gif file in the application you have created it or one of the image viewers.
2. make sure the settings for transprent are aon
3. do a select and copy(Ctrl + C) which will copy the symbol
to the clipboard.
4.open the Symbol manager and click on paste icon.
the symbol should then be pasted in the symbol manager then you can rename it.
I believe the command line version of the SDF loader of
mapguide 6.3 converts the symbols in autocad file to wmf files which will can be brought in directly in mapguide.
Hope this helps. Good Luck!Mohan