But Gatland, who left Best out of his original squad for the Australia tour, admitted the Ireland hooker had since proved him wrong and singled out his leadership since taking over from Paul O'Connell as Ireland captain in January.
Best has led Ireland to a triumvirate of victories against South Africa, New Zealand and Australia this year, the first side to do so since England's 2003 World Cup winners.
"There is no doubt Rory Best has done a really good job," said Gatland. "He looks comfortable. We didn't take him initially in 2013 [to Australia] because we felt his line-out throwing was not good enough. He has obviously improved enormously.
"He is scrummaging well, he is experienced. He has grown into that leadership role. There is a lot to admire about Rory Best. I go back to a couple of years ago when Ulster went over to Northampton and he gave Dylan Hartley a bit of a lesson in forward play and hooking that day. It was another level."
Hartley, meanwhile, faces further scrutiny about his temperament after a year of impeccable behaviour as England captain under Eddie Jones.
It was red card in the Premiership final in 2013 that cost him his place on the Lions tour and opened the door for Best to travel as his replacement.
Meanwhile, The Sunday Telegraph understands that Gatland hopes to add Jason O'Halloran, a former All Black centre and current Scotland backs coach, to his coaching team that was announced in Dublin on Wednesday.
O'Halloran, the former Hurricanes centre who won one cap for the All Blacks against Italy in 2000, has been identified as the second attack coach to work alongside backs coach Rob Howley, who was unveiled alongside Andy Farrell (defence) and Steve Borthwick (forwards) as Gatland's assistants.
The Lions, however, have yet to secure his release from the Scottish Rugby Union, sparking concerns that the 43-year-old will be prevented from joining Gatland's coaching team.'
Gatland suggested on Wednesday that he had effectively been blocked from appointing Gregor Townsend, the Glasgow Warriors head coach, who is due to take over from Vern Cotter as Scotland head coach at the end of the season.
It is also understood the Lions are close to agreeing a tour fee for the players that will see each squad member receive around £65,000 each for the 10-match tour, with a bonus each of £25,000 if they achieve a Test series victory over New Zealand.
Meanwhile, former England fly-half Alex King is poised to join the Wales coaching team on a short-term contract as attack coach for the Six Nations.
King, who stepped down from Northampton Saints in October, will replace Matt Sherratt, who is returning to the Cardiff Blues. King is due to join Montpellier in the summer.
