"He's probably playing some of the best footy he has ever played," said McLean.
The 20-year-old Penrith centre faces 35-year-old Gagai, 15 years his senior, in Sunday's preliminary final at Accor Stadium.
A 338-game veteran, Gagai helped Newcastle reach its first preliminary final since 2013.
He has survived contests with South Sydney's Latrell Mitchell and Warriors centre Leka Halisima over the past two weeks.
Gagai's one-handed offload past Halisima set up Dominic Young's winning try against the Warriors last week.
McLean, who earned NSW State of Origin selection this season, is expected to represent New Zealand at this year's World Cup.
His finals form and big-match experience have earned admiration despite criticism of Brisbane's decision to sign him for 2027.
"I'm in my 16th year of the NRL and he's only just starting so for what he's been able to achieve, it's nothing short of amazing," said Gagai.