Hyperion's Quake 2 port is commercial, and available from many Amiga dealers.
But being GPL, they had to provide source code (Which they did, by sending CD to everyone who 1) owned their Quake 2 port and 2) requested it)
Someone who requested the sources then uploaded them to Aminet, which is of course perfectly legal for code released under GPL.
It would also be perfectly legal for someone to download Quake 2 sources from Aminet, compile them, and upload executable there, but it seems that no-one has done that yet.
(There's also yet unreleased MorphOS port of Quake 2 that's not based on Hyperion's source codes, but as it's unreleased, sources are aswell)