As the term implies, a gentleman behaves in a manner that is non-threatening, and rightly so. In our modern society, there is typically no need for physical contest to achieve social rank, at least not on an individual level. We reserve the physical fight for governments to handle on a much larger scale, war.
Nonetheless, despite what behavior one learns as appropriate, people are still human with instinctive motivations dictating social interaction. With this in mind, I believe it to be critically important for a gentleman to maintain a strong physical presence even though there will likely never be a need to utilize it directly. I liken a strong physique to a nuclear warhead; no one in his right mind will ever likely employ one, but having it commands the respect of others, if for no other reason than for his ability to attain it.