You don't need to heat the whole garage, just one part of the brooder. The other part needs to be cooler so they can move away from the heat source if they get too hot.
We're told chicks need to be at 95°F the first week and the temp dropped by 5° every week until they're at ambient temperature. But the best way to judge if they're too hot or cold is by their behavior. A hot chick will pant and stand with its wings extended. A cold chick will cheep loudly and huddle with the other chicks, trying to burrow in toward the center. They do huddle to sleep, but if they're comfy they will just make soft, contented chirps, not loud distressed screaming. Comfortable chicks will spread out and eat, drink, wander around, explore, play. Good luck and have fun! Welcome to the flock!