If I were you, I wouldn't get another gourami. In my (albeit pretty limited) experience, male gouramis like to be in charge, and they'll bully any other gourami in their space until one of them is dead. With other kinds of (unrelated-to-gouramis) fish, though, they seem to be OK once they establish dominance, and can accept the other fish as "part" of their territory.
Right now I have an opaline gourami in a 20 gallon long, and when he was the only fish in the tank, he *did* seem very bored. He would always try to interact with me whenever I was anywhere near the tank and just generally seemed restless. I don't know if he was lonely, per se, but like their relatives, bettas, gouramis are smart fish, and I do think they prefer having activity around them.
Based on my experience, I would recommend getting a tough and fairly active but peaceful schooling fish to keep him company. Things have worked out especially for me with adding gold barbs. At first, Marcus (my opaline gourami) tried to establish who was boss and got pretty aggressive with this school. I was really afraid for a bit and tried to get him out of the tank, but couldn't, and had to just keep my eye on things and stop him (by pointing at him. Told you gouramis are smart!) from attacking the other fish. After about a day, though, everything had settled down completely. Now the gold barbs *and* Marcus seem very happy. The barb school pretty much sticks to themselves while Marcus does his own thing, but he doesn't fight with them nor they with him, and none of them seem bored or shy. They explore the tank, hang out, just do fish stuff. I'm very happy with the community. Of course, Marcus is an opaline gourami, not a dwarf, and YMMV. Still, I do think that adding a school of fish who can hold their own but who *aren't* related to gouramis can be a really good bet for the fish's quality of life.