Yes, of course it stands up on its own merits, but in practice what makes it useful to the broader open-source movement is as a tool that fills the same gap that Photoshop does.

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.