Wales boss Gatland and his fellow coaches will go into a selection meeting on Saturday evening, following a penultimate World Cup warm-up clash against Ireland.
His squad will then be unveiled on Sunday afternoon, with nine training group players set to miss the final cut.
And Gatland accepts that breaking such bad news is the toughest part of his job.
"It's tough because I know first-hand what it's like to miss out on a World Cup," he said.
"In 1991, I had been involved with the All Blacks and we had just come back from an undefeated tour of Argentina which I had been involved in.
"A couple of months later we came back and I wasn't selected in the squad. I know how disappointed those players who aren't selected are going to be."
Gatland says that "probably 23 or 24" players are currently pencilled in, with Ireland's visit to Cardiff a last chance for some individuals to impress the head coach.
"Having spoken to the players, I think they are pretty well aware of the opportunity on Saturday, and to be fair, they have made a lot of progress in the 12 or 13 weeks together," he added.
"The (training) camps have been fantastic, and we've been very impressed with the players. Sometimes you get one chance and, for some of them, that chance is on Saturday."
Two players in that category on Saturday are fly-halves Jarrod Evans and Rhys Patchell.
Evans starts against Ireland, with Patchell set for a second-half run off the bench. Whoever impresses Gatland most is likely to be Japan-bound - and the other one left at home.
"My sole focus is the game and how I can get the team to perform to the best that they can and how I can get my individual game on the money as well," Cardiff Blues number 10 Evans said.
"We have a strategy of how we want to go about this game, and that will be my sole focus, and take care of Sunday when it comes along.
"It is just trying to perform the best you can and putting your best foot forward. That's all you can do.
"You have got to do what is best for the side. We have always been team first, whatever is best for the team, and anything after that is a bonus."
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