I already have the steam verison of Dishonored 1 and am intrested in purchasing it on gog, but I can't find any information if the Definitive Edition on PC is an improvement of the original Steam release. Any ideas?

I'm not aware of any difference, graphically. It should be the same Definitive Edition from Steam just without any DRM, just as how Dishonored 2 is identical on GOG in that sense (though in D2's case it also includes some formerly Bethesda.net exclusive options which is nice).