Baldur's Gate was first featured in the 1e Cyclopedia of the Realms and first described in detail in the 2e Forgotten Realms Adventures
Far from being created for the games that bear its name, the city of Baldur's Gate has been a part of the Forgotten Realms campaign setting since it was first published. In the Cyclopedia of the Realms booklet of the 1st edition AD&D Forgotten Realms Campaign Set, published in 1987, Baldur's Gate is present and, though it is not mapped out, it gets a couple of paragraphs of description, which firmly identifies its position on the Sword Coast and establishes some of the city's most salient features:
- it is an independent city-state, tolerant but well-policed, thriving on trade
- it ruled by the "Four Grand Dukes" - Entar Silvershield, Liia Jannath, Belt, and Eltan
- as a result of its prosperity it has grown outside the original walls and is now divided into the original "upper" city and the sprawling "lower" city
As well as this specific description, the city is frequently mentioned throughout the Cyclopedia in other entries, since it is one of the most prominent cities of the Sword Coast, and has plenty of history within the region. The Flaming Fist mercenary company, of which Duke Eltan is the leader, is also described in great detail in the accompanying DM's Sourcebook of the Realms, featuring a complete breakdown of their numbers and also full individual descriptions for Eltan and his two most senior lieutenants, Scar and Moruene.
Though the Forgotten Realms setting was first published for 1e AD&D, there are comparatively few sourcebooks for that edition compared to later ones. I don't believe any other published 1e book covers Baldur's Gate in any detail, nor could I identify any Dragon magazine articles which would describe it further.
The Forgotten Realms entered the 2e AD&D era with the publication of the sourcebook Forgotten Realms Adventures (1990), which both updated the setting to second edition and also considerably expanded on its details. In FRA, Baldur's Gate receives its own entry with two full pages (pg76-77), including - as requested - an actual map of the city.
This longer description goes into much greater detail about both the city's lore and its current state than is found in the Cyclopedia, noting the city's fluctuating population of 86,000-115,000 (depending on the season, due to the influx of travelling merchants and sailors), notable individuals and organisations in the city, the history of its namesake Balduran, and so forth. It also describes some of the city's most iconic locations, such as The Wide, the Hall of Wonders, the Elfsong tavern, etc.