I believe the voice assistant can't be activated by voice with Hey Google but once you activate it works fine. Haven't tested Hey Google myself.
Note that voice typing isn't part of Google Assistant. You can get voice typing either by installing GBoard or by installing Google Speech Services. Installing just one of the two is sufficient.
Most Google features, even Pixel specific ones, work on GrapheneOS, as long as the functionality is provided through a Google app, and not the stock Pixel OS itself.