My bad. Yep, you are correct. I thought the GPL actually prevented you to sell compiled version of a GPL FOSS software (Outside of the original maintainer) but it seems it isn’t compared to this one which force you to keep it free. There’s also limitations on what you can remove so yep. Non-free. Seemed a bit Counter-intuitive to me in the first part. https://www.gnu.org/licenses/licenses.en.html
I should probably suggest the GNU Foundation to check this license to compare it, as I often see question of FUTO software license online.
It will never make changes as greater as 20% in the most extreme cases. Those tend to be just pre-configured to maximize such gaming performance, often at the cost of security, compatibility and stability. Most of those stuff could be adapted to other distros, and tend to be actually. If a change or piece of software made such important benefit to playing games, without any huge drawbacks, be sure it will surely be soon integrated in other distributions.