I'm pretty sure I recall that Gibbsy one.I had to look up if Wombat had kicked any other field goals. Turns out he got his first in the final round game of 1978 vs Rorters at the Sports Ground in Manly's 20-10 win. He got the one in the GF replay and then kicked one more in 1979 at Brookie vs Souths in a 15-4 win. Custom Match List - Rugby League Project
1978 was a weird year for field goals as Johnny Gibbs kicked one for a 1-0 lead over the Sharks in the semi final at the SCG. Try that in 2020 and the NRL media and NRL integrity unit would be all over the team.
And I'm pretty sure it would have been his one and only FG too...