Summarize:

After you create still images for each frame of your animation, create a new file in GIMP.  Make sure the the width and height of the file match the width and height of your animation frames.  Use the following steps to open a new file in GIMP.  Click File. Click New  Type the width in pixels next to "Width". Type the height in pixels next to "Height". Click Ok. GIMP creates animations using each layer as a separate frame of animation.  The bottom layer is the first frame of animation and the top layer is the last frame of animation.  Use the following steps to open your image frames as a new layer.  You can select multiple images by holding the Shift key and selecting the first and last image frame:  Click File  Click Open as Layers. Hold Shift and click the first image frame file. Click the last image frame while still holding Shift  Click Open. By default, GIMP exports GIF animations at a frame rate of 10 frames (100ms) per second.  You can add the frame rate in milliseconds at the end of each frame to change the frame rate to something a little faster, you can add the frame rate in parentheses to the end of each frame name in the list of the frames.  For example, if you add "(33ms)" at the end of each layer name, that will give you a frame rate of about 30 frames per second. Before you export your animation as a GIF, you can preview it.  This allows you to see how the animation looks.  If you need to edit any of the frames in the animation artboard file.  Use the following steps to preview your animation:  Click Filter. Click Playback. Use the "fps" drop-down menu to select the Frames per Second. Click the Play button in the upper-right corner. If you are satisfied with how your GIF animation looks, use the following steps to export it as a GIF animation.  Click File  Click Export As. Type a file name next to "Name." Click Select File Type. Click GIF image. Click Export  Click Export again. Click As animation. Type the frame rate (i.e. 30) next to "Delay between frames where unspecified:". Click Export
Create a new file in GIMP. Open each frame of animation as a new layer. Add the frame rate in milliseconds to each layer name. Preview your animation. Export your file as a GIF.