If you go to this link
http://www.haascnc.com/details_VMC_G...8#VMCTreeModel
you will see a picture of a reasonable sized Gantry Router (or Mill). As other posts have explained the gantry is the big bridge structure that straddles the table and moves on guideways down the side of the table. The link has a video download about halfway down the page and you can watch the machine running. Most of the people making their own cnc machines are making little gantry machines. The largest gantry machine I have seen was about 120 feet long and something like 20 feet wide. It was actually built in place and the table was on the floor. It is used for machining aircraft wing components. Gantry machines are often called gantry routers rather than gantry mills and I think this distinction is made because generally they do not have the same accuracy as a mill.