Yeah I get what you're saying about it sounding weird, it's a cultural thing. Where I from tipping isn't required. You pay for what you asked for and if you want to leave a tip for the waiting staff/barista/whoever. You do that as extra, and it's quite a big gesture, it's not expected and it's a way of saying thank you to them for being good whilst hosting you.

But like I said above, in other cultures tipping is not only customary it is expected. So to me gratuity means exactly that, it is expressing gratitude via a financial reward.