That's because the 747 doesn't have a GPS system?
It has an inertial navigation system that should display information similar to a GPS system. For use on this, READ THE MANUAL. I'm sorry but the only way to fly the PMDG 747 realistically and enjoyably, is if you fly by the book using the manual. If you don't they you wasted your money.
It usually takes newbies to payware aircraft sometime to learn how to use the FMC however. Get an online guide for that.

The PMDG 744, like most 744 counterparts, has a dual GPS system.
IRS is also present but is not used as the primary navigation source, it comes behind GPS position and DME-DME updating.
The important function of the Inertial system is to feed pitch, roll and slip information etc to the autoflight system.
Remember that the Inertial System comprises of accelerometers and a stable platform (a gyro), which can sense the acceleration of 1/1000g.
It is this information that is displayed on the PFD/ND.
However there is an option to inhibit GPS and navigate solely by radio/inertial, but there's no need to do that in normal OPS.