Spread spectrum - called Code Division Multiple Access (CDMA) in cell phones. Qualcomm invented it, and still has the controlling patents on it. It is generally a low powered signal that looks like static to a reciever without the key. The low power is why a 4 watt signal can jam so far.
CDMA is a form of spread spectrum. Not all spread spectrum is CDMA. The low power has nothing to do with the modulation. CDMA signals are generally low power, because they can be in the applications used, where line of sight requirements means that distances between transmitter and receiver are short. Your last sentence makes no sense whatsoever. Power density is what is important in denial type jamming. Since a spread spectrum signal is spread over a larger bandwith, the jammer power must be too, and thus the power density, watts per Hertz, will be low for a given total power, it's what makes spread spectrum systems less suseptable to jamming, not more suseptable. With GPS the jammer designer has the advantage of knowing exactly what frequencies to jam and how wide the jamming needs to be, as these are all published standards.