My dimensions are 48x13x21
I would personally not keep more than one
Trichopodus trichopterus in a tank of those dimensions.
Yeah you misread I have a blue gourami but instead of blue its gold
…the common name for a gold
T. trichopterus is "gold gourami" and the common name for a blue
T. trichopterus is "blue gourami". The most common name for this species is "three spot gourami". Regardless of the colour morph, the fish is the same species, unless you are actually talking about one of the other half dozen species which are sometimes called "blue gourami" or "gold gourami".
Again I only have one gourami and I am looking to add another
If your existing gourami is a
T. trichopterus (the colour morph doesn't matter at all for most fish, gouramis included), then "no", we recommend against adding a second one due to territorial aggression. If you add another
T. trichopterus, then there is a reasonable chance that one will kill the other and be damaged enough in the process that it will die of injuries.
If your fish is not a
T. trichopterus, then we need to know what species it is and what species you want to add.