What's this part of the bike, and is it safe to alter it?
What he hammered down is the star nut.  It's a gription (yes I made that word up) device that serves as an anchor point in the steer tube to allow the top cap to properly compress the headset during a headset adjustment.  Old or damaged star nuts are often driven all the way through the steer tube to remove them (that's why your steer tube is open at the bottom).  They are generally only hammered in a short ways because being longer serves no purpose.  In this case, so long as that GPS unit is as stiff as a steel bolt and allows for proper compression during an adjustment, it should be fine.