This probably doesn't apply to TrueNAS, but technically, it's possible to close a GPL project. You'd need the permission of every last contributor to relicense their code, or they'd have to rewrite all the code they can't get relicensed (e.g., someone said no or already died), or they could do it if they never accepted any pull requests because they would then be the sole copyright holder and have the freedom to relicense at their whim.
I can't vouch for TrueNAS, but most open source projects accept pull requests because free labor, whether they're corporate projects or not, so I'd assume they can't freely relicense without a hell of a headache, so yeah, it's probably staying open for the foreseeable future.