^ what those guys said! Always keep your main file a .gal file. and this definitely sounds like jpeg compression, but also bear in mind if youre working with more than 256 colours, and save that as a gif, the colours will be reduced automatically and you dont have a choice what stays and what doesnt! you could be lucky and it just looks a bit weird, but it can practically delete chunks of the picture.
ive run into that whilst making gifs..