Note: gBrowser's help is out of date and incomplete. It may still be somewhat useful however, so here it is.
1. Is gBrowser a universal binary? Is it compatible with Leopard?
Since version 1.6, gBrowser is a universal binary and is in fact ONLY compatible with Mac OS X 10.5 and later; previous versions of OS X are no longer supported.
2. Where does gBrowser store its thumbnails? How much disk space do they take?
Thumbnails used to be stored by gBrowser in an image file's resource fork, but now gBrowser uses Apple's QuickLook API to render thumbnails. Apple does not disclose where thumbnail data is stored, but it appears that QuickLook generally caches thumbnails in an invisible, temporary cache on your boot disk. If used gBrowser back when thumbnails were cached to each image's resource fork, you can remove these old, unused thumbnails by selecting the images (or folders containing images) and choosing "Remove Pre-2.0 Thumbnails..." from the Thumbnails menu.
3. gBrowser ate my hard drive and regurgitated a steaming mass of superconducting magnetic goo. How much can I sue you for?
Please reread the License Agreement. And patent that superconducting magnetic goo, it's probably worth something.