I can't speak from direct experience with the GVP card, but I have used an A2091 SCSI card (Zorro II) in the past in an Amiga 2000. I read several places not to power the hard drive from the power adapter on the card itself, but to use a connector coming from the computer's power supply.
The reasoning was that the power supplied over the Zorro bus was too weak for the later hard drives. It was fine for the intial 40-100MB hard drives but there was not enough juice to reliably power the models that came later.