The 23-year-old is likely to be figure in Sir Clive Woodward's 44-man squad, to be announced on Monday, and Plumtree is expecting great things of his former protege.
"I think Gavin will be a big success in New Zealand," Wellington coach Plumtree told the South Wales Echo.
Plumtree, who guided Swansea to two Welsh titles, a domestic cup final and the last eight of the European Cup, added: "I watched the Six Nations Championship on TV and Gavin has matured into a superb all-round rugby player.
"Defensively he is really strong, which is just as well because he will be up against powerful runners like Tana Umaga and Ma'a Nonu.
"His attacking game has never been in question, while he has the ability to kick the ball 65 metres consistently and can threaten the goalposts from long range as well.
"His temperament is also good. Not many players could have landed that kick towards the end against England, but Gavin made it look easy.
"He and Brian O'Driscoll would make a great partnership for the Lions."