There's probably no way to get the GPS to drive the nav displays or autopilot in the 757, but I don't have it so it may be possible. You certainly can't get it to appear in the VC, not unless you really know what you're doing.
However, what you can do is get it to pop up in 2D.
1) Navigate to [wherever you installed FSX]\Flight Simulator X\SimObjects\Airplanes\[CS757 or whatever the folder is called]\panel\.
2) Open the panel.cfg file in notepad. Look at the top and there will be a section that looks something like this:
Take a note of the last numbered window, in this case 06.
3) Scroll down until you get to the section that starts:
4) JUST ABOVE that line, insert the following code:
And then in the bit at the top where I've written 'XX', replace that with the number after the last one you looked at before, in step 2. So, in our example, you'd put '07'.
Do that and you should be set - the GPS will be available as a popup!