With a 5 gallon you are rather limmited. You could either go for a single dwarf (colisa lalia) gourami or a single honey gourami. You could also keep a trio (1 male, 2 females) of honey gouramies (colisa sota/chuna) if you provide enough cover. Keeping just a male and female isn't a good idea as the male will chase the female and will stress her. Dwarf gourami males are beautiful but larger and more prone to illness and a little more territorial than honeys. Make sure when buying your gourami(es) that you check the scientific name as the common names get muddled up and many different species are given the same commercial name.
You can tell the sex of your gouramies like this:
For dwarfs it's very easy as the male is much more brightly colored while the female is silvery (and becoming difficult to find as LFS seem to only stock males) with a deeper body, wider belly and shorter finnage. There are many color morphs.
Honeys can be slightly trickier. At an LFS the sexes can seem identical but the females are generaly lighter grey-brown to pale yellow in color with a rounded dorsal fin compared to the male's slimmer appearance, longer, pointier dorsal and richer yellow-gold to red coloration. When in good, breeding condition, the males develop a wonderful blue-black underside to contrast with a deep orange-flame colored background.
For a small tank with just gouramies you could use pretty much any filter sold as suitable for that size tank. I actualy preffer small sponge filters meant for use on 10-20 gallon tanks and you don't need an airstone though it would be useful as, though they mainly breathe air, they do appreciate well-oxygenated water (as does any fish).
You should fishless cycle the tank (read link in signature) and provide plenty of hiding places including some floating cover. If the floating cover used is live plants, make sure they don't ever completely cover the surface and obstruct the gouramies' path to the air as they will suffocate. The substrate doesn't matter but a dark substrate will bring out color. They preffer dim to medium-bright lighting but won't mind provided they have cover to escape strong light and regular day/night cycles. A hood is essential as they can jump and need realtively warm air above the water to breath as the cold air outside would likely shock and stress them.
They will show best color when fed a commercial flake supplemented with plenty of live/frozen foods such as bloodworms and brine shrimp. Freeze-dried foods like bloodworms, tubifex, brine shrimp, daphnia etc and fresh foods like cucumber and zuxhinni can also be offered. An occasional shelled pea does them good as well.