Gantry type machine centre was probably the only choice to mill the large sized CNC machined parts. The costs were, needless to say, very high. Sometimes, the component is only long but not too wide and it  could be too expensive to have that component to be CNC machined on gantry type machine center. 

Today, we have better choice to deal with the long profile CNC machined parts with more acceptable costs. The capacity of the machine is X: 6500mm (6.5 meter), Y: 300mm, Z: 300mm. When considering this kind of long profile machining, we have afford solutions now.