Once again, what species of gourami do you have? And is your tank planted?
Also, I assume, as there's only one fish in it, that your tank is new. Is it cycled? Most tetras require a mature tank. Neons, for example, are not very hardy and are only reccomended for tanks that have been running for at least 6 months.
Also, not all tetras are compatible with all gouramies. Some gouramies - such as pearls - are susceptible to being nipped and some tetras - such as serpaes and black widows/skirts - are known nippers.
Besides the huge range of tetras that would work, corydoras catfish, some barbs, livebearers, danios, rasboras, small loaches and even certain cichlids would also work. You have to be more specific concerning what kind of gourami you have and what kind of fish you like if you want accurate suggestions.
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