gThumb is described as 'image viewer and browser for the GNOME Desktop' and is a popular app in the Photos & Graphics category. There are more than 100 alternatives to gThumb for a variety of platforms, including Windows, Linux, Mac, Xfce and Android. The best alternative is IrfanView. It's not free, so if you're looking for a free alternative, you could try ImageGlass or digiKam. Other great apps like gThumb are XnView MP (Free Personal), FastStone Image Viewer (Free Personal), XnView (Free Personal) and JPEGView (Free, Open Source).
ImageGlass is a simple picture viewer with which you will be able to display your image collection in a very similar way to the one that Windows provides by default, but with some additional functions.
digiKam is an advanced digital photo management application which makes importing and organizing digital photos a "snap". The photos are organized in albums which can be sorted chronologically, by folder layout or by custom collections.
FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor. It has a nice array of features that include image viewing, management, comparison, red-eye removal, emailing, resizing, cropping and color adjustments.
HoneyView is a fast image viewer that supports archived image files for comics/manga/photos. Its design is light and the interface skin can be changed. You can set HoneyView as your default image viewer and also add a shell context menu.