I've had a few customers who did their own design work in GIMP. But it's been for really simple spot color stuff. As long as they got it to me at 300ppi, actual print size, it was plenty good enough. (And it's easy to have them export it as some universal file type.)
One of these same customers would also fiddle around with some app on his phone, and mock up text-based designs with really cool fonts. They looked great, but all he could send me was low-res PNGs or JPEGs. I'd have to charge him to redraw in Illustrator. After paying for that a couple times, he went so far as to contact the app developers, & managed to get the names of the fonts. Most were free / open source, so that lowered his art fees. I always thought it was a cumbersome way to do things, but he really liked that app!
That said, it's been 2-3 years since I've knowingly gotten something from any of those programs/apps. Of course, for all the JPEGs & PNGs we still get, there's no way to know exactly what the customers are using.
I actually keep a copy of GIMP around; I never use it personally anymore, but sometimes walk friends & family through it when they want to do something to their photos that less-robust products can't handle. It has a lot of capabilities, it's just not production-friendly.
