I agree with the "overused" comment. Like all dogma overtraining went from unknown to abused as an excuse to not work hard,. yes it real but much overused now.
As for GPP, I say make it fun. Like our DC bootcamps. dragging sleds & trucks, throwing things, swings, hammers. All he hell out we are usually both out of breath and beat to hell but yert laughing.
I have added this as an extra day and really have seen nothing but positive effects from it.
So I say just go out and BUST ass but have fun doing it. It has an Odd effect of not seeming like busting ass. Dan talks about this in the FUN days he does with his Football teams. They will play a game and will soprint jump etc twice as hard and fast as when actually doing sprints but since its FUN they dont notice it.
Hope that helps,