The Collaborative International Dictionary of English v.0.48:

Gobbing \Gob"bing\, n. [See 1st Gob.] (Mining)
   (a) The refuse thrown back into the excavation after removing
       the coal. It is called also gob stuff. --Brande & C.
   (b) The process of packing with waste rock; stowing.
       [1913 Webster]