A 5" long gourami should not go into a 6G tank lol.This means that a 5" gourami should be measured in this manner,
length overall (5"),
thickness, (1/2"),
height, (2 1/2"),
so for this fish you multiply the following, 5x 1/2x 2 1/2, this gives you a total of 6 1/4 gallons of water.
Just use aqadvisor. So much easier and probably more accurate than measuring your swimming fish in 3 dimensions.