In GIMP this is where macros come in handy.
First, you move the 1st GIF to the desired positions. Starting from the side rulers, click and hold on the ruler and the. Slide across until the guide is either on the right most or left most pixel. That's how you create guide lines. Note the X coordinate and remember it for the rest of the sprites.
Next, read up on locking the layers (Godot already stated it)
*Once your layers are in position, each layer must be fixed via "layer to canvas." There is no way to fix all layers automatically, so you need to make a macro for that command to work more efficiently. Use page up/down keys to quickly move between layers. Once fixed, save.
Do note that some sprites, particularly Lang and Shih na's sprites, have widths wider than 256 pixels. You must set canvas size to 256 pxl wide and adjust the centering before doing all this.
*You mustn't skip this step at all. When you move the whole image, you're moving the whole thing (including the blank area) outside the canvas. You will encounter saving problems if you skip this step. In fact, your sprite might not come out as 256 * 192. It might come out as 180 * 190 if not done right.
EDIT: Sorry if I seem like I'm hijacking your guide, Godot... ^^;;;