A new technique that uses a multi-line probe beam to measure the gain on several lines simultaneously was developed. This new technique was used to measure the gains of the peak fundamental lines P$sb1$ (4-9) and P$sb2$(4-9) while lasing on the overtone, for three levels of media saturation. The suppression of the fundamental gains obtained at relatively high media saturation with 99.7/99.7% reflective mirrors was essentially the same as that obtained with 55% higher intracavity flux with 99.8/99.86% reflective mirrors. The gains of the low J lines P$sb1$(4-6) and P$sb2$(4-6) were suppressed 41% to 84%; the gains of the high J lines P$sb1$(7-9) and P$sb2$(7-9) were suppressed 3% to 43%. The 1 $to$ 0 lines were suppressed more than the 2 $to$ 1 lines. The maximum suppression occurred between 2 and 6 mm downstream from the nozzle exit plane, near the center of the 9 mm overtone beam. Simulation of the experiments with a rotational nonequilibrium computer model showed that the fundamental gains are determined by three independent mechanisms when lasing occurs on the overtone. First, overtone lasing decreases the gains of the P$sb1$(J) and P$sb2$(J) lines whose upper or lower levels are directly involved in P$sb{20}$(J) overtone lasing. Second, overtone lasing reduces the rate at which the low J v = 2 states are populated by rotational relaxation and increases the rate at which the low J v = 0 states are populated by rotational relaxation, resulting in suppression of the low J fundamental gains whose upper or lower levels are not directly involved in overtone lasing. Third, overtone lasing reduces the rate at which the HF(0,J) and HF(1,J) states are populated by the various collisional deactivation processes. With 10% of the original rotational relaxation rate, the computer model was in reasonable agreement with the measured zero power gain profiles. The model over predicted the fundamental gain suppression ($Deltaalpha$) for the P$sb1$(8,9) and P$sb2$(8,9) lines whose upper or lower levels were directly involved in overtone lasing, and under predicted the suppression for lines P$sb1$(4) and P$sb2$(4,5). The model predicted the suppression for lines P$sb1$(5-7) and P$sb2$(6,7) reasonably well. With the original rotational relaxation rate, the model was in reasonable agreement with the measured suppression of all P$sb1$(4-9) and P$sb2$(4-9) lines. However, with the original RR rate, the model's agreement with the experimental zero power gain profiles was not adequate.