I understand it is not the most accurate source but I recall a House episode where he claimed the body had a control mechanism for pain in which only the most painful thing was felt. Is that true? and if so how does the body control which injury we feel and could we use this to our advantage?
Pain receptors, more precisely pain signal propagation, can be inhibited in several ways. One is, for example, fine touch receptors (mechanoreceptor in illustration) inhibit propagation of pain signal in spinal cord (a lot of pain processing happens in spinal cord):
That is why humans often rub skin around or nearby injury, whether it is in joints or something more superficial. You should find more information about such control in any textbook on pain. See this page for short primer.
Next, pain propagation is modulated -- still in spinal cord -- by descending pathway from brain and brain stem. See this page for more, or address Kandel's textbook.
Now, I don't know whether or not it is identified how/where brain "selects" to what pain pay attention. My hypothesis is that it happens by competition, e.g. several signals compete by inhibiting each other via some mechanism, probably, related to stress-induced analgesia.
Firstly you have to take into consideration that nociceptors do not adapt to stimulus. This means that a nociceptor will not only fire at the beginning of the stimulus, as seen in mechanoreceptors that have phasic responses, but will fire as long as the stimulus lasts. This type of response is called tonic response (see figure). Figure A shows the tonic response whereas figure B shows the phasic response.
So by this, we can conclude that, peripherally, every noxious stimuli, will induce action potentials as long as that noxious stimulus lasts. During the ascending propagation of the action potentials induced by the noxious stimuli, other neurons will have the "opportunity" to inhibit the response. Some ways that the pain transmission can be inhibited is by the release of endogenous opioids by descending neurons from brain parts in mid-brain and medulla.
The other mechanism is the one explained above by aandreev. And as you mention in your title, this mechanism is called the gate control theory.
Ref: Purves et al (2012).,Neuroscience 5th ed.,Sinauer associates. http://www.ucl.ac.uk/anaesthesia/StudentsandTrainees/PainPathwaysIntroduction [2015-04-30]