I am not a graphic designer but I do some small graphics work occasionally and I use GIMP and Inkscape.
GIMP and Inkscape are both very nice and quite powerful - but if you compare GIMP to photoshop its obvious GIMP is not on the same level (I've never user Illustrator so I can't compare it with Inkscape)
Photoshop has more features, produces better results, has more plugins and virtually all graphic designers know how to use it - it's also very expensive.
For the same amount of work you will have better results in Photoshop - that means that if you use GIMP you will either produce lower quality work or need more time.
So that all boils down to how much money quality and your time worth - if you only do simple work GIMP can do well or you are an hobbyist and your time is essentially free than you can't compete with GIMP's price - but if you need a powerful tool and your time is expensive (and if you have any success as a professional graphic designer than your time should be expensive) GIMP doesn't look so free anymore.
And that is without taking into account you might someday need some plugin that is only available for photoshop, need to exchange PSD files with other designers or hire employees and have to train them to use GIMP.
If my main job was graphic design I would definitely get Photoshop as soon as I can (maybe do a few small projects with GIMP so I can afford it).