The Collaborative International Dictionary of English v.0.48:

-gen \-gen\ [(1) From Gr. -gen-, from the same root as ge`nos
   race, stock (see Genus). (2) From Gr. suffix -genh`s born.
   Cf. F. -g[`e]ne.]
   1. A suffix used in scientific words in the sense of
      producing, generating: as, amphigen, amidogen, halogen.
      [1913 Webster]

   2. A suffix meaning produced, generated; as, exogen.
      [1913 Webster]

Jargon File (4.4.4, 14 Aug 2003):

gen
 /jen/, n.,v.

   Short for generate, used frequently in both spoken and written
   contexts.