You may be correct, that would mean that the name is bit confusing. Sorry, didn't test it out, but I see 'progress bar' and that is for me not something that is going left and right. My bad.... should have checked.
In the days when SWF preloaders were able to be used the AS3 code inside the SWF preloader file was able to do some calculation to determine the download percentage. An animated GIF file does not have that ability. So all you get with these GIF files is an animation. If you wanted a readout percentage of the download, that would need to be done somehow in code. I'm not sure if it's possible with client-side JS alone or whether it would require server-side scripting as well.