The Kiwis' Cup plans were turned on their head by North Queensland wrecking ball Jason Taumalolo's last minute defection to the minnows.
Tonga are now considered a World Cup dark horse with their ranks also bolstered by the likes of Cronulla juggernaut Andrew Fifita who backflipped days after being named in the Kangaroos squad.
"Tonga will be a danger in the competition, for sure," Gillett said.
But Gillett believed New Zealand should not be dismissed as a Cup threat.
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"I don't think they have been weakened, they are still playing for their country," he said.
"Their players will want to do their country proud, they will still want to win.
"I don't think they will be weakened.
"I think they will use it (defections) as motivation."
Taumalolo's shock move was reportedly in protest over coach David Kidwell's decision to not consider Jesse Bromwich and Kevin Proctor for the World Cup after they were embroiled in a cocaine scandal following May's trans-Tasman Test.
In contrast Fifita claimed he wanted to honour his "father's heritage".
Kangaroos captain Cameron Smith admitted he was shocked by Fifita's call.
But Gillett said he had no problem with it.
"Obviously you want him to play beside you but he is playing for his father and I respect that," he said.
"That's up to the individual to make up their own minds."
Gillett wasn't letting go of his green and gold jersey any time soon.
"These things don't come around too often and your rugby league career can be pretty short," the seven Test backrower said.
"This could be my only opportunity to play a World Cup.
"I don't want to look too far ahead, but to win a World Cup on Australian soil would be an outstanding achievement."
