Bettas are good gouramies for small tanks but you could only keep a single male and perhaps a few shrimp (depending on the betta's tolerance) or some kind of aquatic snail (apple is ok but they get pretty big and produce a lot of waste). You can learn more about bettas in the Betta section (they are given special treatment

- away from the rest of the gouramies) if you are not already familiar with them. There are also several wild betta species that you could probably try out but it's best to begin with
Betta splendens. There is a huge number of different colors, patterns and fin types to choose from if you are prepaired to buy from a breeder/order online and still a good selection even if you go for a pet store fish.
If you want a more 'typical' gourami, a single male honey would be ok in a 6 gallon. I'd heavily plant the tank and throw in a few shrimp (perhaps 3-4 cherries? Check out
http
/www.petshrimp.com ). There are quite a few color morphs to choose from but make sure you have the right species -
Colisa chuna/sota; not
Colisa lalia (which isn't as hardy).
A trio of sparkling gouramies (not croaking - they grow a tad too big) - if you can make sure of the sexes - would also work in a heavily planted tank.
Not many other common ones would work, to be honest- and including more fish tankmates is not a good idea.
Some of my favourite gouramies for small tanks are the licorice gouramies such as
Parosphromenus alfredi, P. deissneri and
P. nagyi. Many of these are miniscule and peaceful but, unfortunately, also quite fragile and intolerant of less-than-ideal conditions. The most common is probably
P. dayi (which is a little big for a 6 gallon) or
P. deissneri. If you are prepaired to put some research into it, these fish are worth looking into. One big problem you may face is identifying them so make sure you are getting them from somewhere reliable.