The first spectroscopy results for the gas giants were obtained in the first decade of the 20th c.
However it would take many more decades of technological improvements and research developments for our modern understanding to emerge. Even now we have lots of unanswered questions, such as whether Jupiter had water present in its atmosphere in the past.
http://www.geochemsoc.org/publications/geochemicalnews/gn142jan10/atmosphericchemistryoftheg/