Seems to be some confusion about this, further promulgated here. I don’t know when this changed, but in recent Marshmallow gapps you need to set your default app for “Assist & Voice Input” to the Google app for this to work.
Settings>>Apps>>Gear icon>>Default Apps>>Assist & Voice Input
Select Google App, and all is well. Now in Google settings, Now On Tap toggle will be flippable. Turn it on and you’re good to go.
I think I had to reset this after upgrading my rom, or maybe just after clearing cache. Anyhow, it works again. It’s one of those things you don’t figure out until you need it in a pinch. Handy feature.