O'Driscoll, 34, was a shock omission when Gatland announced the team for Saturday's Test series decider against Australia in Sydney. New Zealander Gatland, who as Ireland coach first capped O'Driscoll in 1999, has preferred Welshmen Jonathan Davies and Jamie Roberts to forge the centre pairing.
While O'Driscoll's Ireland career is set to continue next term, his four-tour Lions odyssey has reached an abrupt end. When the Lions tour New Zealand in 2017, he will be 38. Reflecting on the O'Driscoll decision, Gatland said: "It's only hard because you are making the decision by using your head and not your heart."
He added: "Then you realise that what comes of making a decision like that is all the peripheral stuff - not the rugby decision - because it becomes a major story for 48 hours and it becomes a debate.
"That is the process I've gone through myself. If I go back to the UK after this and say 'did I make the decision because I believe it's the right decision?' or 'did I make the decision because it was the right political decision or sentiment?'
"I have to put hand on my heart and say it's the right rugby decision. I would hate to think we had made calls on trying to avoid criticism or public favour or perception.
"He (O'Driscoll) is not finished yet. He has been a big part of the story (2013 tour). He played in the first two Tests and he has been a part of a win and a loss.
"He is obviously very, very disappointed, as any player would be, but it's like everything, it's a learning process. It's kind of hard when you've been the number one in your position for so long, for 15 years, and first choice on every team you've been a part of, and on every Lions team you've been the first choice as well.
"There have been a lot of people under him who have experienced the disappointment of Brian O'Driscoll always being selected over them. It's just part of sport, isn't it?
"Sometimes it happens and we've made a really tough decision, a tough call and that's part of it."
- Sports & Recreation