I'm not exactly sure what you're asking.
You have individual panels that need to be combined? Just copy/paste them into the same image. When you paste something in, look at your layers dialogue. There will be a new layer called Pasted Selection, or something similar. If you rename the pasted layer, it will become its own 'real' layer, and you could then paste another panel in. If you need to resize the amount of space you have, use Image > Canvas Size. If you need to resize individual panels, use Layer > Scale Layer.
Thank you very much!