Assuming the hard drive is compatible with the Apple IIGS (check with
MicroNet) here's what you need to do:
Make a new bootable System Tools disk, using the Installer from the SCSI
utilities disk, and specifically install the SCSI driver from the new
utilities disk that came with the SCSI card. This makes the disk realize
there is a SCSI device attached. As the system starts up, it looks for
SCSI devices and if it finds one that should be readable but is not, it
will then ask if you want to initialize.
Make sure the slot the SCSI card is installed in is properly set in the
Control Panel. (That slot should say "your card".)