Guild Wars 2

For a birthday surprise, I’d like to send a gem store item as a gift.
I have no way of knowing what store items they already have without asking them. Asking would be fishy so close to a birthday, spoiling the desired effect.

So if I were to gift them an item from the gem store in the hopes they don’t have it, and I find out I was wrong, what then?

It’s pretty easy to ask these things off-handedly to get the information out of your friend without alerting them to your motive.

That said, you would have to open a customer support ticket and ask for a gem refund if they already had it unlocked. I don’t know if that works in the case of gifts, but that’d be your only option.