According to gogen-allguide.com デマ comes from the German demagogie, so it would make sense for it to be similar to demagogy in English. But demagogy is used for political things like false claims and promises, where デマ is much broader nowadays to mean any groundless statement or rumor 「根拠のない、いい加減な噂話」, even if it was political in nature at first.
For ガセ gogen-allguide.com simply defines it as 「偽物。まやかし物。噓。」 and describes it's origin as probably being the がせ of お騒がせ.
These are both very general definitions, so are they completely interchangeable anymore or are there situations where one would be preferred over the other?