Actually they say a 55g is minimum size for a single adult O and 75 for an adult pair minimum. I personally recommend 75 for singles and a six foot 125 for pairs. Not so much because of space (which is also important) but more because of the amount of water held. Oscars are notoriously messy, so the more water volume, the less issues you'll have with nitrates/nitrites.
As far as giant gourami go, you could cram one into a 7 foot tank (200g) if you wanted to but a 25-28" Giant G would occupy nearly half that space given little room to swim. I personally would never keep giant gouramis unless I could provide AT LEAST a 10 foot long tank that's 4 foot wide or better. This would be giving a 25" gourami 4 times the lengths of its body to swim, and just under 2 times it's length to turn. So let's say you do a 120" long, 36" tall, 48" wide aquarium. That's just under 900 gallons which is a phenomenal size. 10x4x3ft. Exact gallons for a fish this size isn't the issue, it's the swim space that it really needs. Life long keeping of a giant gourami isn't for someone without deep pockets IMO.
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