gThumb is described as 'Image viewer and browser for the GNOME Desktop. It also includes an importer tool for transferring photos from cameras' and is a very popular Image Viewer in the photos & graphics category. There are more than 100 alternatives to gThumb for a variety of platforms, including Windows, Linux, Mac, Flathub and Flatpak apps. The best gThumb alternative is IrfanView. It's not free, so if you're looking for a free alternative, you could try IrfanView or XnView MP. Other great apps like gThumb are ImageGlass, nomacs, Sly and FastStone Image Viewer.