Gatland admits that performances during the Six Nations have already had a significant impact on selection but that there is still plenty to play for before he and head coach Ian McGeechan name their final tour squad late next month.
"Wales versus Ireland was a big game for players in both squads," said Gatland, who saw his 2008 Grand Slam winners slip to fourth in the final standings after a nail-biting 17-15 defeat at the Millennium Stadium.
"There are players in the same position who are 50-50 calls at the moment. So Wales versus Ireland was pretty important.
"Some players we had pencilled in have fallen away with their performances, but we still have the quarter-finals of Europe to look at. There is still a lot of rugby before the team flies out.
"This week will be key for some players, but realistically there is a bit of time before we have to name a squad."
When the time does come to announce the travelling party for the 10-match tour of the Republic, Gatland has suggested that selection will be based on a player's ability to adapt to South African conditions and on their mental attitude towards a unique team effort as well as their form over the closing weeks of the season.
"Part of the thinking is about looking at South Africa and the conditions," added Gatland.
"In a 35-man squad you need players who are suited to the hard grounds and the high veldt. So we have to make sure selection isn't just based on the Six Nations. You may have players sitting on the bench for teams who would be suited to a tour there.
"For a lot of players, it isn't just their rugby ability which is going to be important but also their character.
"It will be about how they handle themselves off the field and how they cope with the disappointment of not being selected. You don't want somebody who goes away and sulks.
"How players respond to that will be key. You don't want a split in the camp."