Like others said, I use Scale Image to reduce the size. If it's something that I want buyers to be able to zoom in on, I do 1000 pixels width and allow height to scale in proportion. If it's something I don't want them to zoom (like a print), I do 570 pixels width, because that's how wide Etsy shows photos in the listing.
If you don't need the original photo (as in, you are just editing the photo and want to save it as the same file) you can select File > Overwrite
I feel your pain... I lost my editor program when I had to get rid of Windows and move to Linux, and Gimp has quite a learning curve.