Gorgeous fish! Don't get horribly attached to his coloring as that might change. Most fish with patterns have marble backgrounds and are likely to change. What will he be sharing the 20 gallon with?
Becareful with pairing gourmanis with bettas since they both are surface breathers. Both species are territorial and can be aggressive to each other. Rams are a hit or miss as well since both species are aggressive. I personally would not recommend a betta with either species.
I've had other bettas with them, so I know he'll be fine. It's just getting him to stop being so mean, I find with aggressive bettas it usually takes 1.5-2 weeks to integrate them into a tank with others. I've had much more aggressive bettas then him and after the 2 weeks they're very peaceful