Well, it's the exact opposite for me. I don't do a lot with Gimp but I can only remember one time I needed to google how to do something, I just find it very intuitive to use. Sure it's not the most polished software and I've seen one or two bugs but overall it does everything I need with an easy to use interface.

> Changing it in the filename box returned an error

I always do it like that and it just works. Though in current versions you have to "Export" to jpg/png and not "Save as". Afaik the intention is to prevent overwriting the original file on accident.