No one's claimed that 'developing in public' is a prerequisite of being Open Source. It is, however, usually a result of it. The source code being publicly available is a prerequisite. And since it's easier to update a GIT repo than to maintain a public and private branch, in the vastly overwhelming number of cases, Open Source stuff is developed in public.
Since is it Open Source you can download the source code and compile it yourself. In most cases, as in the case of GIMP, you also have access to the development code and can download and compile that. Therefore it is perfectly fucking acceptable to say, particularly in a specific situation where it is the case that, since it is open source, you can get it.
If it were proprietary you would not, in the vastly overwhelming number of cases, have access to the source code and be free to do this. I was making a distinction between common methodologies, not quibbling about licenses like a twat.
I remember reading something about a UI plugin for GIMP that rearranges the menus and buttons so it mimics Photoshop (assuming you're more used to that).
Might help?