The bombs are laser guided not GPS guided . GPS jammer won’t do squat also a GPS jammer is a active transmitter which means that it can pin pointed & killed by a homing anti-radiation missile.
For the Small Diameter Bomb:
http://www.boeing.com/boeing/defense-space/missiles/sdb/
Guidance:
Precision inertial navigation system/global positioning system (INS/GPS). Anti-jam GPS and selective-ability anti-spoofing module (SAASM).
“If the GPS signal is jammed or lost, the JDAM can still achieve a 30 meter CEP or less for free flight times up to 100 seconds.”
http://en.wikipedia.org/wiki/Selective_availability_anti-spoofing_module
Selective Availability Anti-spoofing Module (SAASM)
SAASM does not provide any additional anti-jam capability, because it uses exactly the same signal-in-space as current GPS signals (power and modulation). Future GPS upgrades, such as M-Code, will provide anti-jam capabilities.
Deployment of the next generation military signal for GPS, called M-code, commenced with the launch of IIR-M and IIF satellites, beginning in 2005. A complete constellation of 18 satellites with M-code capability is planned for 2016.