Once Gast learned that Jenova wasn't an Ancient, and rather a demonic threat from the past, he got out and tried to persuade Shinra to stop. Hojo, though, wanted to press on. I'm guessing that Gast was around when Sephiroth was conceived, though, even if that particular experiment wasn't his idea.
Sephiroth spoke highly of Gast several times, almost like Gast was a departed "father figure" for him.