Daniel Sergant was a performer, puppet builder and proffessional dancer who worked for henson from the late 1960s and early 1970s. I do not know exactly when he joined or left.
The only time that Daniel Sergan performed Gonzo was in the 1970 special The Great Santa Clause Switch, where Gonzo was known as Snarl, the cigar box frackle.
Daniel Sergean also performed Big Bird in a few episodes of Sesame Street during the shows first season. He performed Big Bird because Caroll Spinney was sick at the time. I do not know whetehr Daniel also performed Big Birds voice or if Caroll dubed it later, but Daniel Sergeant also performed Big Bird in a 1969 appearance on The Ed Sullivan Show, possibly for the same reason. In that appearance Big Bird didn't speak at all, probably because he wasn't performed by Caroll Spinney, although if Daniel Sergan did perform Big Birds voice, then I don't know why he couldn't have had Big Bird talk.
I think Daniel Sergan also performed in The Frog Prince and The Muppet Musicians of Bremen. I don't know if he performed any voices in those specials, and I don't know what a Daniel Sergan voice would sound like.