I've tested XnView and it definitely exhibits the gamma error described on the page. It would be a great if XnView added the feature to use gamma correct scaling. It could even be made the default if desired. The examples on the page are very compelling.Technically speaking, the problem is that "the computations are performed as if the scale of brightnesses was linear while in fact it is an exponential scale." In mathematical terms: "a gamma of 1.0 is assumed while it is 2.2." Lots of filters, plug-ins and scripts probably make the same error."
The incorrect method is shown first:
Generally it seems more detail is preserved with gamma correct resizing and the gamma is obviously correct.
Please consider this feature. As argued by the site XnView would be one of the few softwares that maintains correct gamma when resizing.