afaik you can modify the linux kernel... and release it, but you can't call it the linux kernel, you would have to call it something else, it would be a fork, and you would have to include credit and copyright (or left or whatever) crap giving credit for what parts you didn't do

however im not a legal export nor an expert on the GPL.. i've read it a few times, but like any legal document does anyone _really_ understand what it means?