Not necessarily, no. There is a long history of people releasing closed source drivers, particularly on embedded platforms.
So far as I can tell the kernel devs think it's harmful and A Bad Thing (TM), but not a violation.
Also as has been pointed out below, it wouldn't automatically make it GPL. It would make it in violation of GPL and releasing under GPL would fix the violation, but it wouldn't make it GPL.
There's a 10-year-old post to LKML from Linus saying the GPL nature of kernel modules is a huge grey area. I don't think it's been resolved since then.
So far as I can tell the kernel devs think it's harmful and A Bad Thing (TM), but not a violation.
Also as has been pointed out below, it wouldn't automatically make it GPL. It would make it in violation of GPL and releasing under GPL would fix the violation, but it wouldn't make it GPL.