The kernel remains GPLv2, so the standard licensing issues apply: you can make changes, but if you distribute the changes, you have to provide an offer to distribute the source to the changes (and follow through with it). Those changes are then licensed under GPLv2 as well. LKMs are an exception, where code which does not use GPLv2-only symbols can remain a proprietary binary blob. (GRsec is too tightly wound into the kernel to be an LKM)