I think Riley could definitely kill a human. He's a soldier, after all. That kind of training is part of his job, and if he weren't assigned to The Initiative it's likely he'd be working somewhere else, possibly killing people or at least involved in the killing of people.
But Gunn could kick his ass. 😛 Just saying! 😀 Even if Riley has more technical training, Gunn's fighting style is a lot more spontaneous and stylistic. You need all of that to be a good fighter! Riley's good, but Gunn's better. It kind of reminds me of what Buffy says to Kendra in Season 2. Kendra is well technically trained, but Buffy has an edge to her.