if gd is already enabled and it still wont show, i'd look at the mu side next. the guild mark isnt a png, it's a raw blob stored in the guild data and the cms redraws it with gd pixel by pixel. two things that catch people:
- the guild never actually set a mark ingame, so that blob is empty and nothing renders. test with a guild you know has one set.
- those draw scripts usually assume a fixed byte length for the mark. if your files store it a bit differently the script can hand back a blank image without erroring at all. flip display_errors on for a moment and load the rank page, if gd is choking on the blob length you'll see it straight away.