Thaiday scored the only try of the opening hour, while Gillett, in his first game since picking up a shoulder injury in State of Origin III, tallied a quality 81 metres and 43 tackles in a powerhouse outing.
The Dragons remain just two points outside the top eight but face an uphill battle to make the finals.
Marshall's hopes of earning a new contract also nosedived along with the Dragons' season after the veteran committed five of his side's nine errors in a disappointing performance.
The 31-year-old twice fumbled balls in attacking opportunities, bombed one kick out on the full and also crucially was caught on the last tackle with the Dragons in search of the game-winner late in the match.
In torrid conditions in a dull first half, both sides opened conservatively, tucking the ball under their arms and completing at a rate of more than 90 per cent in the dreadful conditions.After trading penalty goals, Thaiday's 23rd-minute chasedown of an Anthony Milford grubber gave the visitors an 8-2 lead by halftime.
Dragons winger Kurt Mann almost scored a long-range try but fumbled a pick-up with the tryline begging in the 52nd minute, before opposite flyer Lachlan Maranta went over eight minutes later.
Broncos coach Wayne Bennett was delighted with the input of Gillett and Thaiday, as well as a third star from suspension in prop Josh McGuire.
McGuire chipped in running 167 metres and making 47 tackles.
"You have to understand that's half your forward pack and they're all current Australian players," Bennett said post-game.
"They're not on the way up in football, so taking 50 per cent of them out of your team has a huge impact. They complemented everyone else out there."
Dragons coach Paul McGregor conceded that, with four rounds to go, the Dragons' season was over.
"Gold Coast having a draw the other night makes it hard for us because of our for-and-against," he said.
"You don't like to put pressure on, pre-game, about having to win games of footy because then you worry about the outcome before you worry about the process going through it.
"But it makes it difficult. It's a good opportunity now to give some of these young players that are going really well and get that experience back and finish the year strong."
- AAP
