Why "state-sponsored" attacks specifically? If any group of attackers is targeting me, then I'd be just as concerned. Introducing that distinction seems like it will force Google to determine whether a group is backed by a government.
There's many other warnings and notifications that Google gives users about phishing and hijacking attempts. This specific warning is exposing the fact that we think someone is being targeted by a government backed attacker. We believe this is information that could be useful for some people and that we shouldn't keep this to ourselves.
I'd guess from a network level "state-sponsored" attacks are easier to fingerprint than individual password phishing attempts.
I imagine this relates to certain attacks which are only feasible for Governments.