I'm afraid I wont be of much help here since I use photoshop.
But in Ps you can create a new image, then convert the background layer into a smart object which lets you erase the solid background. 1 layer png images does also in most cases let you erase and create transparency directly.
I'm very certain that if you don't find it by clicking around inside of gimp, someone else will be able to tell.
Have you tried oh so wonderful google? Should have the answer xD