Agree with others. Gourami can be difficult, but there are a couple that would suit a 20g. The Honey Gourami
is one, and there is the pygmy Sparkling Gourami
depending upon water parameters. You can read up on these in our profiles; click the shaded names. Profiles in general are under the second tab fro the left in the blue bar across the top of the page.
Tetra need a group as someone mentioned, and space in a 20g is limited as some of these tetra can get 2 or 2.5 inches in length. There are other colourful characins similar to tetra but a bit smaller and perhaps more suited to a 20g, in the pencilfish group. These are under characins in the profiles. Water parameters can be critical, as most of these will be wild caught, as explained in the respective profile.
There are also some small fish among the Cyprinids; these are similar to characins in that they are generally peaceful, need a group, etc. Some do better in soft water, some in basic medium hard. You'll have an easier time finding suitable fish once you know the parameters of your tap water.