Best answer by MikeV
Google Play Music is not available through Alexa, and probably never will be. Google and Amazon are very hostile towards each other lately, specifically over Youtube and Amazon Fire TV devices, though it's spilled into other areas of both companies' businesses. Google likely has little desire to develop anything for Alexa given that they have their own voice assistant, and Amazon has their own music service, along with support for a number of other large players, so they're likely of the mind that they're not missing anything by not having GPM.
Eventually, when Sonos adds Google Assistant support, I'm sure we'll get the ability to use voice control to get music from Google Play Music too.