I'm just surprised they would impose that restriction in the kernel considering they have very little problem with closed-source binary blobs for drivers and whatnot.
Unfortunately, I don't believe the high level list of which hooks have gpl helpers is published, so reading the code is the best method currently.
Where GPL comes in is when you use BPF to introspect the kernel. It is quite possible to use BPF programs with the kprobe hook to extract data about how the kernel itself works, and the opinion of the kernel developers on this is that such use cases are within the realm of GPL protection.
That said, lawyers have certainly given their approval to the way Nvidia packages their driver, so it should be fine or at least very very hard to challenge.