I only sell online and have no counter. Do I still need the POS add-on for gift cards?

Yes, after the trial. The Gift cards, Store credit and House accounts screens belong with the counter, so they are greyed out with Needs the POS add-on until the add-on is on. They are fully open while your free trial runs, so you can set everything up first. Gift cards and store credit also sit with the loyalty features in your plan. Selling a card on your ordering page is a menu item with a tick, but the card itself, its balance and the Outstanding figure live on the Gift cards page. Settings → Plan & billing shows the add-on price and what it covers.