You don't need to collect more heart pieces for Gyorg. To beat Gyorg, just stand in the middle of the platform, and move back there every time he rams it. When you can, shoot him with an arrow. If it hits, he'll be stunned. Quickly put on the Zora Mask, and swim over to Gyorg. Zap him with the Zora Shield, and then quickly swim back to the middle platform, and time it so that when you breach the surface you land on the platform.
I suppose that you could try and paddle back to the platform, but this runs the risk of Gyorg catching up to you and chewing on you.
Now, I find Zelda II to be a hard game. Knowing how to use your shield correctly makes or breaks you.