McDowell, who remains in the battle to make the 16-driver playoff field, ended up with the fastest average lap speed of both sessions at 98.515 mph.
Gilliland, however, went off course with less than three minutes remaining in the second 20-minute session and got stuck in the gravel trap. His No. 38 Ford did not appear to suffer any significant damage.
Eight of the top-10 overall speeds came from Group B, including the top three.
Bell led the way in the first 20-minute session with an average lap speed of 98.258 mph but tagged the wall off Turn 14 on his final run on the track.
It was unclear what – if any – damage Bell sustained to his No. 20 Joe Gibbs Racing Toyota.
Bubba Wallace overshot Turn 1 midway through the session and had to serve a stop-and-go penalty before continuing his run.
Former Formula 1 world champion Jensen Button was 14th, Japanese sports car star Kamui Kobayashi was 15th and former LeMans 24 Hours champion Mike Rockenfeller was 19th fastest.
McDowell quickly went to the top of the second session with an average speed of 98.515 mph.
Larson was second quick (98.436 mph) and Gilliland was third (98.402 mph).
Ty Gibbs and Bowman rounded out the top five.
Three-time Supercars champion Shane van Gisbergen, who won the inaugural Cup Series Street race at Chicago last month, ended up 12th-fastest in the second session.
Fellow Supercars star Brodie Kostecki stalled on the track on his first lap before eventually making it back to pit road so his No. 33 Richard Childress Racing team could address the problem.
Team officials believe Kostecki’s car had an issue with the throttle pedal. He eventually returned to the track and ended up 38th-fastest.