Wow... I never heard of glue-ing down gravel over a whole yard. I know they sell bonsai trees with the soil covered in glued on gravel and people complain about how hard it is to get off.
It would seem that it could only be glued to the top inch of soil. Can't you just dig under it and lift it off? I am visualizing starting from an edge and peeling it off sort of like stripping sod, but of course since I have never seen the stuff I don't know what I am talking about.
What a terrible thing to do to what could have been a nice little backyard - hopefully with your effort, eventually it will be again.