otherwhirl: i don't think you understood ofnuts, actually
. i'm not entirely sure i understand you either, but i'll give it a try! here is how i understand you: you have one image with lots of layers. you have put those layers in layer groups and named each layer group with numbers. am i right so far? you now want to move each layer group to a new image but you get problems pasting the layers in at the same place on the canvas (it's a bit confusing before you've learned what everything is called in gimp - i think you mean canvas not palette? palette would be a tab where you manage colours, canvas the actual space within an image).
if this is what you want to do, there are a few tricks. to begin with, you can add guide lines to the new image. if you want the tshirt centered in that image you can make two guides under image->guides->new guide and place them in the exact center of the canvas. then you can make sure that the "snap to guide" under the "view" menu is checked. when you paste in a new layer in the image, it will snap the center of that layer to the center of the canvas.
if you don't need lots of empty space around your tshirts, you could go to image->autocrop image, make sure the original tshirt layer is selected and do an autocrop. this will give you an image with no transparent borders, and the center of the tshirt layers will automatically be the center of the image. go through all your layers and do a "layer to image size" and you will have all the layers centered in the same place. when you make the new image to paste them to, make sure that image is the same size as the original, then you will find all the layers paste in at the right place.
there are other ways too, i just don't have the brains this early in the morning to describe them.
i wonder, though - if you're just moving the layers to a new image to save and export that image, wouldn't it be easier to just do this for every new mock-up: go layer->new from visible and then copy that visible layer and paste it in as a new image? do you really need each mock-up saved as an image with all the layers?
there's of course always the possibility that i've misunderstood you completely - i need a few more cups of tea to get reasonably intelligent...