The Giff and Space Jockeys are two completely different races. That's a fact. However, I believe the space jockeys are the those who the Giff told about; the ones who came on their ship and put the alien eggs on there.
Page 236 from Steel Egg:
"...Those Who Come From Beyond the Galactic Rim, the Place of Darkness. We admitted some of them to this ship, to negotiate with them, a peace mission. But while they were here, they planted the eggs of the creatures you call xenomorphs on our vessal. Many hundreds were killed. Their bodies were committed to space--- others we had no time to gift to the galactic mother."
^being said by the only surviving Giff.