The Collaborative International Dictionary of English v.0.48:

Gaseous \Gas"e*ous\ (? or ?; 277), a. [From Gas. Cf. F.
   gazeux.]
   1. In the form, or of the nature, of gas, or of an
      a["e]riform fluid.
      [1913 Webster]

   2. Lacking substance or solidity; tenuous. "Unconnected,
      gaseous information." --Sir J. Stephen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gaseous
    adj 1: existing as or having characteristics of a gas; "steam is
           water is the gaseous state" [ant: liquid, solid]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "gaseous":
   adulterated, aerial, aerodynamic, aerostatic, aery, air-built,
   airy, attenuate, attenuated, chimerical, cloud-built, cut, dilute,
   diluted, dreamlike, ethereal, fanciful, fatuitous, fatuous, fine,
   flimsy, fuming, fumy, gasified, gasiform, gaslike, gassy,
   gossamery, illusory, imaginary, insubstantial, mephitic, miasmal,
   miasmatic, miasmic, oxyacetylene, oxygenous, ozonic, phantomlike,
   pneumatic, rare, rarefied, reeking, reeky, shadowy, slight,
   smoking, smoky, spirituous, steaming, steamy, subtile, subtle,
   tenuous, thin, thinned, thinned-out, uncompact, uncompressed,
   unreal, unsubstantial, vaporing, vaporish, vaporlike, vaporous,
   vapory, watered, watered-down, windy