The Lionel GS4 Daylight looks identical to the Bachmann model-is it a rebadged Bachmann product?
Not exactly.
When Lionel decided to get into HO for the 3rd (or was it 4th time?), they contracted with Kader in Hong Kong (Bachmann's owner) for several models, including the GS4, GP30 and others. When Lionel decided to again get out of HO, Kader had the tooling, and released most of the models as Bachmann with little or no changes.