When Gerber is selected as the Export format, you will receive 3 additional options before ACE outputs the Gerber files.
Neg - Output specific Gerber file with opposite polarity. - i.e Negative Layer to Positive, or Positive Layer to Negative.
Mirror - Flips about the Y Axis to create a mirrored Gerber file.
Drill - Round flashes found in this gerber file will be used to generate an NC Drill file. Has the same formatting options as the Gerber file.