The Gully Catwalk went in at the same time that they piped Slalom Gully. It was at least a couple of years before HV went in, because what is now Imagine was Harper's Gully, and (looking from the bottom of Slalom Gully) there were trees all along the bottom of Imagine.(about 3/4 the way, under where the chair is now). The trees were removed when HV went in.
And Harper's dropped into Slalom Gully after it was piped. So my guess is that the Gully Catwalk went in around 1984.