Widnes ended a run of three straight defeats with a 42-16 triumph.
Gentle said: "It was disgraceful and the most inept performance I have seen for a long time.
"We weren't anything like a top-four side out there and are kidding ourselves if we think that. We have a lot of work to do.
"We had no respect for the ball and no defence to back it up. It was embarrassing and not up to standard."
Hull's top-four hopes took a big blow with the loss, while the Vikings remain bottom but are now level on points with London Broncos and Castleford Tigers.