Hartley called Barnes a "f****** cheat" during Northampton's Aviva Premiership final defeat by Leicester at Twickenham on Saturday and he has been replaced by Ireland's Rory Best in the 37-man squad for the tour of Hong Kong and Australia.
"Dylan's a young man who has made an unfortunate error," said Gatland at Monday's departure press conference in Kensington. "To play the game you have to play on the edge, but unfortunately he's gone to the edge of the cliff and jumped off it. That's the only analogy I can draw."
He added: "I called him yesterday and left a message on his phone. He left a message on my phone today thanking me for the call.
"I was looking forward to working with him and really feel for him. He's made a poor judgement. Hopefully he can put this behind him and move on.
"It's my understanding that he won't be appealing the decision, but that's entirely up to him. He may decide to do that in the next day or so."