The Collaborative International Dictionary of English v.0.48:

vitamin G \vitamin G\ n.
   same as vitamin B2.

   Syn: vitamin B2, riboflavin, lactoflavin, ovoflavin,
        hepatoflavin.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

vitamin G
    n 1: a B vitamin that prevents skin lesions and weight loss
         [syn: vitamin B2, vitamin G, riboflavin,
         lactoflavin, ovoflavin, hepatoflavin]