Yeah....VERY bad plan. Theres about a 98% chance of your betta going after the Gourami, the Gourami going after the betta, or both. Gouramis are from the same family and can be just as aggressive as any betta, in fact it was an Opaline Gourami I had that whipped out practically my entire 50 gallon a few years back, I've never had a betta that aggressive. Not to mention is highly likely that your betta will mistake the Gourami for another betta and go after it like theres no tomorrow.
You might be able to get away with it if you have a VERY large, densely planted tank...but your going to run into some serious issues in a tiny 10 gallon where there isn't nearly enough room for each to establish their own territory.