no...
does not have a differential lock
this differential is same as in a car
if one wheel breaks traction the other tries to pull
if you get stuck the only way out is to get some other machine or something as big and pull it out (or push)
:o)

i'm suprised it got stuck though
generally the GP series is designed to run on non-paved ground hence the P for pneumatic tires.
Unless you accelerated too fast and broke traction and didnt stop when you first noticed it and kept trying to go.
Pneumatic's are not impervious to loose gravel though, they can get stuck too ;o)