Another option that is fairly similar to "gthumb" is xzgv It is an X-based 'successor' to zgv, which was the "classic" image viewer for the Linux console.
"Most file formats are supported, and the thumbnails used are compatible with xv, zgv, and the Gimp."
With its lurid name, I somewhat hesitate to mention pornview. It is as good for displaying "less lurid" material, though you might want to rename the binary if showing it off at work or to your mother...
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