He made Gardner a WR because he was the second best WR on the team. We had nobody behind Gallon. Bellomy was good in practice and proved he was reliable enough if Denard got injured. When the lights shined, he didn't show up. It happens but you can't predict that until the moment.
We weren't going to leave our second best WR on the bench in case our starting QB got hurt, that would be hedging against your own chance to win. Denard going down and Bellomy not stepping up allow your hindsight to kick in, but entering the situation Hoke and his staff did what any staff would do.
FWIW, Gardner was always the backup QB as a WR. If Denard went down just for a game, Bellomy was coming in. If Denard went down for the season, Bellomy would finish the game and Devin would take over for the remainder of the season. That was the plan all along and it (unfortunately for us) came to fruition.