Gap is the driver's best lap in a session against the fastest lap in their class in that same session (zero is fastest in class), shown as time lost and as a share of the lap, so conditions cancel out and tracks can be compared. Typical is the median session.
In GCR, Section 9.4.5.E.4.b., change table as follows: "Over 2699 2700-3999 lbs. 1.50 x .120 1.75 x 0.095 2.00 x .080 4000 lbs. and up 1.75 x .120 2.00 x .095"