The 17-year-old Gilliland shot a 4-under 66 on the Atlanta Athletic Club's Highlands Course to take a one-stroke lead over Casey Wittenberg of Memphis, Tenn. Wittenberg, also 17, had a 69.
Jerred Texter, 16, of Millersville, Pa., the first-round leader after a 69, was one of 78 players in the 156-player field who failed to complete the round. The field will be cut to 64 for match play Thursday after the completion of the second round.
Play was suspended for two hours because of lightning and rain, and darkness forced the suspension of play at 8:45 p.m.
Defending champion Henry Liaw, 16, of Rowland Heights, Calif., shot a 71 to join Zac Reynolds, 17, of Edmond, Okla., at 2 over. Reynolds shot a 67.