I would say that, based on using Photoshop on various colleague's machines, its learning curve is quite a bit steeper than GIMP's; whether GIMP's is steeper than that of Elements, I cannot say, as I'm too cheap to pay for Elements (much less Photoshop), and haven't had any opportunity to try it.
I will say that if you know one image manipulation application, the learning curve for the others gets a bit shallower (but may have a few "gotchas"). (I'm also familiar with a fairly old one, Lemkesoft GraphicConverter)
I do use Adobe Illustrator (and bought a very out-of-date version thereof, for my Mac Performa).
All of the "prepress" I've done both for my handful of (outsourced) edible printing projects, and for my Christmas cards since I got my color laser printer, has been done in GIMP, on my G4 "Bionic Desk Lamp" iMac, with the Christmas cards printed from Preview.