You can usually steal a parking space in the rear building of Post Biltmore (right next to Cypress Pint & Plate/behind the Gordon Biersch on Peachtree. Alternatively, Calhoun & Ethel are close to the NW side of campus. Best free parking really depends on where you need to go on campus. Park on Ethel in Homepark.
https://www.reddit.com/r/Atlanta/comments/pc72p/free_parking_around_georgia_tech/