Google's messaging strategy is famously terrible, but its approach to music isn't much better. Google is moving slowly but surely to make YouTube Music its only music service, and that means more signs that Play Music is being mothballed. The option to purchase Play Music gift memberships is gone in the latest version of the app, and the webpage version probably isn't long for this world.
Before the most recent update, you could purchase Play Music gift subs in the settings under General > Send gift. The option is gone now, but it used to present one, three, and six-month subscription options. They cost $9.99, $29.97, and $59.94, respectively. So, no discount for buying more gift months.
Before and after the update.
Buy & send digital gifts
Important: Google Play Music subscriptions can no longer be purchased as gifts.
Don't fret if you just gifted someone a subscription. Google will probably begin migrating everyone to YouTube Music before long, and subscriptions should carry over. Presumably, Google will have gift memberships for YouTube Music eventually.
- Google Support