The iconic sporting venue, which is owned and operated by the WRU, underwent a recent £3m refurbishment, including a re-design of the hospitality facilities.
Lewis said the revamp would provide greater flexibility to entertain VIP guests at Wales's forthcoming autumn encounters as well as appealing to a range of new clients looking to host conferences and events in Cardiff all year round.
"The Millennium Stadium sets the benchmark in terms of the on-pitch experience and the refurbishment ensures a fantastic off-field experience to match.
"The stadium has witnessed an extraordinary past 12 months, it has attracted virtually every major artist who has toured the UK and witnessed the success of Welsh rugby. It has become a must-visit stadium.
"We have made a significant investment in the stadium and the facilities, something that hasn't been done since the stadium opened in 1999 and it will raise the bar of our hospitality offerings.”
Gatland added: "In Wales we are proud to have some of the best rugby facilities in the world.
"The Millennium Stadium is the best rugby stadium and the best event stadium and has provided me with some very memorable experiences.
"There is nothing to match being part of the atmosphere here with the roof closed and the crowd singing. For a squad it's great to be part of.”
The biggest investment in the hospitality facilities since the stadium first opened in 1999, aims to improve the sport and entertainment experience for corporate hospitality clients.
Completed in time for this autumn's Dove Men Series, which kicks off when Wales host South Africa on November 9th, VIP guests can watch all the best international rugby action from the comfort of the stadium's first-ever pitch-facing lounge or choose from 112 designer suites and five lounges, catering for between 12 and 200 people.
The project was completed over two phases over a 12 month period. Phase One developed 30 hospitality suites in time for Wales to clinch the 2013 RBS Six Nations title, while the overhaul of a further 87 suites was completed over this summer.