This 22cm Giant Gourami was removed from the Oscar tank because it harrassed one of the Albino Oscars to the extent the the Oscar was in permanent hiding and had no more scales left on the one side of it's body....I think it was just a personal "thing" between the 2 of them, as he left other much smaller fish and the other Oscars alone...
So, Generally I'd say he should not be a problem to your Lungfish, but then,.... I have no experience with lungfish and cannot verify that it will be fine with your GG...
Thank you both very much for your replies and taken into account of what you both said ive bought the African lungfish and its getting on fine with the Giant gourami