The Collaborative International Dictionary of English v.0.48:

gum tree \gum" tree`\ n.
   Any tree that exudes a gum, such as:
   (a) The black gum (Nyssa multiflora), one of the largest
       trees of the Southern States, bearing a small blue fruit,
       the favorite food of the opossum. Most of the large trees
       become hollow.
   (b) A tree of the genus Eucalyptus; a eucalypt. See
       Eucalpytus.
   (c) The sweet gum tree of the United States (Liquidambar
       styraciflua), a large and beautiful tree with pointedly
       lobed leaves and woody burlike fruit. It exudes an
       aromatic terebinthine juice.
   (d) The sour gum tree.
       [1913 Webster]

WordNet (r) 3.0 (2006):

gum tree
    n 1: any of various trees of the genera Eucalyptus or
         Liquidambar or Nyssa that are sources of gum [syn: gum
         tree, gum]