So first open the Image 1 in Gimp.
Now go to "Menu > Open as Layers".
Select the Image 2 to open in GIMP. We are takng only two images here, but you can open as many images as layers to add in the animation.
After adding necessary images, select "Menu > Save As".
Now choose to save the image as GIF Image.
When the "Export File" dialogue appears, select "Save as Animation" and click export.
Now adjust the options such as delay between frames and save.
Now you have made a GIF animation with GIMP. You can make as much as you want and enjoy creating them. Fortunately, since GIMP is cross platform, this does not limit you to only Windows or Linux.