There was GPIB hardware connected to the Macintosh. It could have been connected to the serial ports or to a NuBus card. The manual is dated 1994-1995, so that would put it in the System 7.5 era.
There were a few companies making A/D and DIO boards for Nubus Macs (have a few) plus GPIB. Don't know if anyone made PCI boards specifically for the mac but I assume PC boards with the correct drivers would work.