Thought I'd add another little tip about the "cheat"
Ideally, you want it as small as possible, so it won't show much of a "halo", but just enough larger than the throbber icon to blend the jagged edges away. For circles, I've had better luck achieving this by starting out with a large circle, and resizing down, then using that behind the throbber frames. This lets you get a cheat thats only fractionally (a fraction of a pixel) larger than the throbber image itself. I usually have the throbber image smaller than the canvas, also, to allow room for the cheat.
For example, I might start out with a cheat that's a 114px grey circle in a 128px image, and scale down.
Sometimes too, you can get good results by making a copy of the real throbber layer, then Layer > Colours > Brightness-Contrast, reduce contrast completely to give a grey image without detail; then boost brightness. Make a few copies of that layer and test by making a gif copy of the image. This is good to try for non-circle shapes.
Testing is easier if you have a test html page, with background set to something like #c7c7c7