If Baldur's Gate 3 has given you a hankering to play tabletop Dungeons & Dragons games set in the titular city, D&D Beyond has you covered. The Baldur's Gate Gazetteer is a digital supplement full of info about Baldur's Gate, from its geography, to the gods that are worshipped there, to the dark secrets it holds, to new rules for your character backgrounds that better represent seasoned locals.
You can claim it completely free by heading over to D&D Beyond, and peruse it at your leisure using the site's handy wiki-like layout.
It's a really robust bundle of stuff. It doesn't include a pre-written adventure, but there's tons of fodder for creating your own stories in the city. I like that there's more than just pure lore—it includes relevant stat blocks, maps, and random tables, and is sprinkled with plenty of useful rumours and adventure seeds to build on.
The Baldur's Gate 3 connection is stronger than you might realise, too. This Gazetteer was originally included in the Descent Into Avernus campaign book. As well as featuring some action in Baldur's Gate, the campaign actually serves as a direct prequel to Baldur's Gate 3, letting you play out the hellish fall of Elturel, experience the Blood War that Karlach fought in, meet the arch-devil Xariel, and more.
It has its flaws as a campaign—playing through it a couple of years ago I found it quite unfocused, with lots of jumping around between different styles and tones—but playing Baldur's Gate 3 I have been pleasantly surprised how relevant many of its events ended up being to the game's backstory.
If that sounds appealing, you can pick up Descent Into Avernus for $30 for on D&D Beyond. But if you'd rather come up with your own Baldur's Gate campaign, you should be absolutely set with this Gazetteer for no money at all.