The Mail on Sunday newspaper today features an interview with former employee Helen Long, whom The Yorkshire Post spoke to in March, and another personal assistant named only as Paula.
It comes after Sir Gary resigned from the tourism agency, which is partly public-funded, after an expenses scandal.
Paula told the newspaper: "He became a monster, but the board and senior management allowed it, saying, 'You know what Gary is like'."
Ms Long, who worked in the film industry before joining Welcome to Yorkshire, said: "The demands of film stars, studio executives, directors and top actors were nothing compared to dealing with Sir Gary.
"He spent money like a drunken sailor and lived like a rock star thanks to W2Y. The truth needs to come out."
Welcome to Yorkshire chairman Ron McMillan also resigned last month amid the scandal.
Sir Gary was not available to the paper for comment, but friends suggested the £40,000 was repaid as a result of a compromise agreement with the agency and is not an admission of guilt.
All his expenses were incurred in the course of expected professional duties and were signed off by the chairman and senior colleagues, the paper reports.
A spokesman for Welcome to Yorkshire said it has recently appointed two independent professional services businesses to undertake separate investigations into expenses and the culture, governance proceedings and management of the organisation, which are hoped to be completed by the end of June this year.
Sir Gary is seen as instrumental in bringing the Tour de France to Yorkshire, the legacy of which can be keenly felt this weekend as the Tour de Yorkshire race takes place in the region.