Yeah, Gravatar is one of those companies that offers a “free service” where you feel like you’re the user but you’re actually the product. They have the capability of tracking your movement and behavior across multiple sites and aggregating and selling that information. So, gravatar has been jettisoned.
Discourse’s old default avatar system generated a unique icon based on your IP address. It was abstract and visually distinguishing, but not terribly informative—you could tell “red squiggle person” from “blue squiggle person,” but that’s about it. The new system @sam implemented uses the letter of your username against a unique background (so two "R"s, for example, will have different backgrounds). The letters aren’t the visual wtf soup that the IP address-based geo-shapes were—they’re classy.
Anyway, if you don’t like yours, it takes all of 3 seconds to fix it with a custom image in your profile