I actually found it to be quite difficult to sort through old, and outdated sites, there seem to be quite a few on Gadget out there. Here are the best I have so far found.

Inspector Gadget Net
Definitely the best Gadget site I've been able to find. This place has the works and is both easy on the eyes and for navigation.

Animation Cartoon Factory
This site is fantastic. It has work boards from the original Gadget episodes, and character and prop designs for various episodes. The page is maintained by the artist Brian Lemay, who worked on the Gadget series.

Inspector Gadget Page
A great personal site with general info, galleries, etc.

The Case files
The Case files of Inspector Gadget has excellent episode synopsis plus additional trivia information on the series. Not all episodes are up, but are definitely in the works.

Toonopedia - Inspector Gadget
A wealth of cartoon information, links to infinite toons and character links between differing programs. Great Gadget information, but try out the search and make use of the excellent archive here.

Animation Archives - Gadget
Another informative site by the Bandwidth people.

Hidden Mickeys in Inspector Gadget
A curious place, I'd never noticed any of these before. For the curious and observant.