My question, is it realistic to switch and would I be able to easily create graphics like this?
Yes and yes - you would be able to do that sort of thing (maybe with slightly poorer text rendering), however since you are used to Photoshop already, and (presumably) own a legitimate copy (which isn't cheap!), why switch to Linux?
You could, for example, keep Photoshop on a Windows 7 partition and dual-boot. Or you could just not switch to Linux - if you are a graphic designer who already owns a legitimate copy of Windows 7 and of CS5.5, I don't see a real reason.
If you're doing it because you are tired of paying for upgrades, consider that GIMP may be comparable in features to a 10-year old version of Photoshop (CS1). It doesn't, for example, have adjustment layers yet, which Photoshop had since CS2. Just in case you were worried that CS5.5 is getting out of date...