GMM does help with that, but you have to have good SNR in each panel and remove any LP gradients beforehand. it probably helps to use dnaLinearFit to get the panel brightnesses similar before attempting the stitch.
http://www.astrobin.com/full/70295/C/?image_list_page=2&nc=
is a 3x2 mosaic made using this technique. it helps that the image is Ha in terms of lessening the gradients but i still had to remove them.