The spot broke last September ahead of the Rugby World Cup in the UK and compared what the former Welsh international had to face on the pitch with his battle with his sexuality.
Thomas says: "Telling them [his teammates] I was gay was the toughest thing I’ve ever done. But when I needed them the most they were there for me."
The Glass Lions were introduced last year to honour work that sought to challenge gender inequality, imbalance or injustice.
The US and India are leading the way this time with four nominations in the shortlist each. The winners will be announced at an awards ceremony at the Palais des Festivals in Cannes this evening.
India’s nominations include recognition for P&G India’s "Share the load" for Aerial by BBDO India featuring an older father apologising to his grown-up daughter for not setting a better example by helping her mother out with housework.
M:United, McCann Worldgroup’s agency for Microsoft, earned one of the US’ nominations for "Make what’s next", which sought to encourage girls to study STEM (science, technology, engineering and mathematics) subjects by highlighting how few inventors are women.
Last year’s Grand Prix went to "Touch the pickle", an ad for the Procter & Gamble sanitary brand Whisper that sought to address the taboos around women and girls’ periods.
In 2015 work from UK agencies won two Glass Lions – Leo Burnett's "#Likeagirl" and FCB Inferno's "This girl can" – from three nominations. The awards were not divided into gold, silver and bronze.