I do use GIMP myself, I have only ever taken 1 deepsky capture although taken a few unguided widefields. When I have the time I may try to look at the option of using GIMP instead initially. I have just had a load of work come in so a bit busy with that ATM.
One thing in Photoshop is 16 bit whereas GIMP is only 8 bit TIFFs. I use Infraview to try retain the 16bit format while converting. I haven't checked to see if GIMP converts the opened image from 16 to 8 bit which would be drastic.