What's mean "to despicable sub-John Gielgud gestures of amazement" ?

John Gielgud was a famous actor. 'sub-John Gielgud' would refer to an amateur or unprofessional performance. I think what the expression refers to is over-the-top, very obvious gestures of amazement. The point is to emphasise how surprised people are when someone who doesn't meet the media image of celebrity shows talent.