Doing Graphics is not my main job description, far from it (somewhere between damage control and janitor is more like it), but I still do a lot of front end development, at my job I prefer to do all the graphics production work myself for the apps I'm building and have tried using GIMP a crap ton of times over the years - I've never found it to be a suitable replacement. Maybe its familiarity but I've honestly tried to use gimp in this role and just getting around has always been a complete nightmare compared to Photoshop. It may be able to do the job that Photoshop does, but nowhere near as easily.

Having our Devs at our shop use Photoshop is an expense, but like good chairs and big ass monitors, at the end of the day a few hundred bucks every few years isn't really that much and if its that much easier to use then its an expense we'll eat and happily. This also means from our devs perspective the choice between PS and Gimp is not a financial one, and once you take money out of the picture most folks are going to choose PS. So anyway, no I don't think this is a Photoshop Killer.