I wouldn't go so far as to say that Gio is terrible. I think it's more that PDB is having Gio play in situations he shouldn't be playing in; PKs and that last minute 6 on 5.
He's bad. He brings virtually nothing to the table. There is no sugarcoating it. The advanced metrics bear this out. He's terrible at face-offs, and any time he's out on the ice, the Devils are getting out-shot, pinned in their own zone and he just floats around hoping for his wingers to do the dirty work and keep the puck possession in gear. For anybody who says he should be moved to wing, doesn't realize how awful he is along the boards and the fact that because of his size, he can't win any physical battles.
Deboer can be as stubborn as he wants to be, and maybe a part of it is that he's got no other alternatives. But his insistence on playing Gionta up on the lines is holding other players down like an anchor, his matching of Gionta against Stamkos tonight was mind-blowing in it's idiocy. When Josefson gets healthy, Gionta needs to be waived or healthy scratched the rest of the way and he shouldn't be brought back next year either.
We can do significantly better than having him in the lineup, that's for darn sure.