The IIGS, if I recall, has its internal drive circuitry set for slot 6 by default. (So does the Apple IIc with its internal drive). This is because most people who bought the disk controllers for the II/II+/IIe put them in the sixth slot (if I recall the manual encouraged the use of that slot) and most programs had defaulted to slot 6 for disks by then (including, I believe, ProDOS).

If I were you I'd put it in a different slot. Your mileage may vary with reconfiguration, but I'd put it something else to minimize conflicts.