At least on Mac OS X, if you want to copy text from one source and paste it into Gimp inside a Gimp text tool text region, you’ll find that this doesn’t work with the usual Control-V (Command-V on the Mac) keystroke. In short, the secret solution is to right-click inside the text tool area and select “Paste” from the popup menu. I have no idea why the normal copy and paste approach doesn’t work in Gimp, but I do know that this approach works, at least with Gimp 2.8 on Mac OS X.