Yes, it's possible. I made my avatar that way. All you have to do is: Open the first frame of animation. Then click File<Open As Layers. Select the pictures representing the other frames of animation. After that open the layers dialog (Ctrl+L) and arrange them in animation order. Next, save as a GIF file. You will be prompted to either save as an animation or flatten the image since GIF files can't have more than one layer unless it's an animation. You may also be asked to convert it to grayscale or indexed clolor modes if your current color mode is RGB*. Convert it to indexed.
Or you could have just Googled "Gimp Animation".
*RGB color mode is all of the colors, grayscale is black and white, and indexed is where you choose from a range of colors on a platte, and can only use those colors.
