GIF isn't proprietary anymore. The patent expired on August 11th, 2006 and became a free format on October 1st of 2006. It's just an inferior format to (A)PNG and GNU has decided to exclude it even to this day.
For very simple graphics (with few colors) with transparent background GIF format is very useful to generate small-sized files. If you have a website with high traffic, those Gifs can save you a lot of bandwidth. Try it yourself.
Other thing is that unfortunately IE6 (that many people still use) do not support natively PNG's, so GIF has to be used to support transparency.
Anyway... I think that if gThumb supports opening and modifying GIFs would be good to have also "Save as GIF...". I'm not sure how difficult is to achieve that function but if it is too complicated then, I think is better just use GIMP for that purpose.
Vahan Harutyunyan(Brainstorm moderator)
wrote on the 13 Aug 11 at 19:25
Please file a Wishlist Bug against gThumb (EoG) for this issue.
Closing in Brainstorm.