A slight note on GIMP's interface, in the next major stable version (2.8) it'll have a semi-major overhaul by making tool windows dock-able. It will also solve on of the biggest flaws in it by introducing layer groups, thus allowing for more organization. Besides that, Photoshop and GIMP are incredibly similar, regarding the interface. It is really easy to, for example, grab a Photoshop tutorial and use it on GIMP, even if making things differently to achieve such purpose.