I meant use GIMP to actually make the GIF
* Open -> Open as Layers
* Put them in order
* File -> Save As...
* Filename.gif
* Export as animation
* Set to replace frames and set frame delay
Bam
You can change the frame delay for each separate frame by adding (#ms) to the end of the name of the layer that frame is on
i.e. "Layer 2 (1000ms)"