They keep about 3% of the money raised for all donations. And there are transaction fees for withdrawls too! Plus, you have to be registered as a non-profit or there are all sorts of legal red tape issues that your district can get into. GFM is a for-profit business entity.
There are companies that specialize in school fundraising, such as SnapRaise or DonorsChoose, but I've never used them. I have friends that have, though, and I would recommend them over GFM