Gainly & Chalant

Shouldn't gainly and chalant be words? How can you be ungainly if you can't be gainly? How can you be nonchalant if you can't be chalant? I mean, it's common enough to find things that are innocuous, but other things can be nocuous. We've all heard of the placebo effect, but there is also a nocebo effect as well. I'm not sure we should allow words that seem to exist only in contrast to something else if we don't have the something else by which we define their opposition.