You can not gift ESO Plus, because it renews if you don't cancel it. You would have to log on their account, buy three months of ESO Plus and then cancel it. But this is not safe unless you are good friends or family.

You can buy a Summerset upgrade as a gift – on this site, for example: https://www.greenmangaming.com/games/the-elder-scrolls-online-summerset-standard-upgrade-pc/.