The PG7601 is capable of running Negative Gauge setpoints using its Differential measurement mode. This requires using an RPM (RPM1 thru RPM4) to read current atmospheric pressure which gets subtract out as the PG7601 is floating an equivalent absolute pressure.
The recommended RPM4 range to use would be the A100K. This will provide the least additional uncertainty and can be a premium or dedicated RPM4-BAROM model for the best results. An RPM4 with other Absolute ranges can also be used (Ex. A200K range) but would add additional uncertainty. An RPM3 can also be used as well as a few other 3rd party barometers.
The basic concept to operate in Differential mode is that the PG7601 will be switched to a different SETUP profile where the high accuracy RPM4 is selected to be used for the barometric source. The first step is to determine the barometric pressure offset. This is accomplished by connecting RPM4 COM1 to PG7601 COM2 using a standard RS232 cable. The RPM4 will be plumbed to the PG Test port as it will float a pressure close to atmospheric pressure, determine the current error in the RPM4 pressure value relative to the PG, and store this value in PG memory for temporary use. The RPM4 calibration coefficients do not get change. The RPM4 is then disconnected from the Test plumbing and the Negative Gauge/Differential mode DUT is connected. You can then set a pressure such as -5 PSI directly from the PG terminal and it will display the actual pressure that includes the the temporary offset and compensates for fluctuations in atmospheric pressure during the testing as the RPM4 remains in communication with the PG. The PG7000 Operation Manual has related information in section 188.8.131.52
See the attached document for the procedure to operate in this measurement mode.