You have to decompose the spritesheet. In the main GD ribbon, search for “Image converter >> Decompose spritesheet”, it will generate the sprites as individual images, then import them in GD 
Finally, if there are few frames, you can add them one by one in the animation. Otherwise, if there are a lot of frames, you can add them as described here (there is a bug about the order of the imported frames, it’s reverse, the bug is solved for the next release of GD, now disponible as test for Windows): [url]2 super simple features to make gamedevelop much more user f]