"Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins."
Their goal seems to be a good tool, not an alternative to any other software.
And it works well for me! Thanks Gimp.
Precisely :)
It's just like LibreOffice/OpenOffice, of course these tools stand on their own merits too, but again the reason they're important to the open source movement, i.e. what makes them useful in promoting open-source, is that they fill the same gap in the market as Microsoft Office.
Speaking generally, the approach to pushing open-source in practical way is to build open-source tools that cover the key functionality that commercial software offers (as well as functionality that commercial software doesn't yet offer) in order to provide people with less of a reason not to switch.
In other words, Gimp is an "alternative to Photoshop" when framing it as part of the open-source movement (which is what I had implied with my earlier comment). This is not the only framing that can be applied.