I seem to recall making animated GIF from Preview.app on earlier OS, but can't find the options to make a GIF on the latest version of the app.
Can I use 10.8 Preview to make animated PNG or GIF?
If you hold down alt/option when you click the format dropdown in the export menu, you can still select GIF in Mountain Lion. Once you have done this, you can continue with the normal animated gif creation process:
Since Lion, the Save/Export to GIF (and some other formats) has been hidden away, you need to option-click the Format menu in the Save dialog to find it.
I tried to follow these directions for creating animated GIFs, but I wasn't able to add more than one frame. Perhaps with a bit more persistence that will help though, I may have missed something in my quick attempt.