Hi,
It is ages, since Ruby was relicensed:
https://github.com/ruby/ruby/commit/2cd6800fd8437b1f862f3f5c44db877159271d17
Now I just realized, that on licenses page, there is link to LICENSE.txt, which probably changed at that time as well, but I am not sure if the rest of the information is still valid, e.g. "Compatible if dual licensed with GPL, otherwise Incompatible"
V.