Man, Google, Yahoo and Bing exist for a reason! Anyway, here's what you're looking for:
http://www.romhacking.net/utilities/619/
Far better than GC ISO Tool from my experience. Only thing I'd say would be to extract the ISO in the program itself rather than in Dolphin, as Dolphin puts the System Data folder in the wrong place and GC Rebuilder won't be able to rebuild it. Simply load up your ISO, right click the disc icon with "root" next to it and hit extract. When you're done making changes, just open the "root" folder in GCRebuilder and hit Root --> Save, and then Rebuild. Simple.
Best of luck in your modding endeavours!