It's not really about the tool you use, it's more about the ability to produce the end product to the quality that is expected, whether you use GIMP, Photoshop, CorelDraw, PhotoPaint, even Maya, it's all irrelevant.
Studios tend to use programs specific to the needs for the end product. Here's a simple breakdown:
Gimp: Free, and good, but very limited.
Photoshop: Used to manipulate photos, people use this for graphic design in its entirety and I wholly disagree with this use. Fancy effects, graphics, certain elements, but only for elements and photo retouching. Excruciatingly slow and resource intensive on large projects. Outputs overly large files. If you save any pngs, use tinypng.org, to shrink them down.
Fireworks: Best for web products - simpler effects, and more accurate, with vectored rasterised output and it creates significantly smaller project files. Fast and nimble, works great for web.
Illustrator: Usable for one page, maybe two page projects, or compositions. Fancy effects on text are usually imported and placed within a compound or clipping mask. End file is usually small, with a bunch of supports. Bad for text-based and print solutions that do not involve a die cut. Fast but poor when handling a large amount of type.
InDesign: Great for managing large print projects, bad for web. Filesizes are small, and it has a great package utility which allows end users to send a project and have InDesign do the leg work and gather all the fonts used and images in a structed way for the printer. It's fast. And great for type. Easy to use even in an advanced way. Very familiar to Word.
CorelDraw: You won't get much brownie points with this, I'd veer away, however some print studios do use this, but it's a bit of a botch job tool.
QuarkXPress: A dead tool. Overpriced and outperformed by InDesign, it's rarely used as a tool of choice.
PhotoPaint by Corel: Good, but just for that, it's best used for paint/creative and imaginary results. Not much more.
Maya/SoftImage/3DSMax: Not used for any of the above. 3D compositions, and very hardware resource intensive. You'll need to know your algorithms for this too.