A gem card is a card you can buy on amazon or some random games shop which holds a key for usually 2000 gems. Your friend can just enter that code and get the gems.
Also I think you can gift buy gems directly, though not sure if I don’t mix something up here.
You can check in-game at the Currency Exchange part of the Trading Post. If you click on “Custom Exchange” at the bottom, it’ll let you put in how many gems you’d want to buy (200 or 300, whichever the price is for the living story eps each), and it’d show you how much that would cost at that time. You could send that amount in gold to your friend.
Gem cards are gift cards you can buy online or in certain stores (my local Walmart and Best Buy carry them, but some GameStops and Targets also carry them) that have gem amounts pre-loaded onto them. The person who gets the card scratches off the back to reveal a code that they’ll enter into the Gem Store in-game.
