Google Play credit is the product
Balance on the Google account pays for the subscription along with apps, games, films and other subscriptions billed there. There is no separate step where the recipient assigns it to video.
Because of that, the gift keeps its value even if they cancel — the balance stays and buys something else.
The iPhone complication
If the recipient subscribed to this service through an iPhone, the Apple account bills them at a different price, and Google Play credit cannot touch it.
This catches people constantly, because the service is Google's but the billing is not. Have them check which account shows the charge.
If it is Apple, you want Apple credit instead — see Apple Music for how that account works.
Family plans
The plan owner is billed. Credit on a family member's account does not reach the owner's bill. Check who pays before buying.
What the subscription includes
Ad-free viewing, background play, downloads, and in most markets the music service bundled in. Worth knowing because a recipient who only wanted music may already be getting it here.
Region
Google accounts are country-specific and credit is too. The account country is tied to a payment address and is not casually changed, especially with a balance sitting on it.
Asking for one
Give us the country and which platform bills them. Related: gift cards for subscriptions and the music guide.