In one of the closest calls of the XV announced by Gatland for the Cape Town Stadium showdown, the Scotland international edges out Adams for the second wing spot alongside Anthony Watson.
Van der Merwe has used his power and footwork to torment defences throughout the tour and gets the nod ahead of the Welshman whose haul of eight tries was not enough to secure his place in the starting XV.
Now the rivals for the left wing face different challenges – van der Merwe coping with the backlash awaiting him against the world champions and Adams recovering from the disappointment of being overlooked.
Van der Merwe was born and raised in South Africa and played twice for the Junior Springboks in 2014 but qualified for Scotland on residency grounds having joined Edinburgh three years later.
The bulldozing 26-year-old’s switch of allegiance has seen him targeted by trolls on social media and Gatland insists he can expect a similar reaction on the pitch on Saturday.
“It’s a challenge for him. I’ve spoken to him about what’s coming on the weekend and he’s fully aware,” the Lions head coach said.
“I presume every time he touches the ball, they’ll be trying to hit him with everything they’ve got and there will probably be some verbals as well. So he’s well aware of what’s at stake and he’s been incredibly relaxed.
“Duhan has scored some really good tries on this tour and has beaten a lot of defenders and brought some physicality.”
Gatland has revealed that finalising the back three, which is completed by Stuart Hogg, was the hardest selection puzzle of all.
At the heart of it was the difficulty of leaving out the prolific Adams, but the coaches’ minds were made up when a couple of minor injuries were considered alongside the life-changing events of last week.
Josh Adams was a late withdrawal from the starting XV to face South Africa ‘A’ a week ago to witness the birth of his first child on Zoom and when he returned to play the Stormers last Saturday, he struggled.
“It was a really tough call leaving Josh Adams out. I spoke to him and said it was such a tough call for us and for himself,” Gatland said.
“He had a pretty emotional week last week with the birth of his child and then the next couple of days checking in.
“By his own admission, he probably didn’t play as well as he normally would have done against the Stormers and that’s completely understandable because of the week he had.
“He had a knock on his hip and quite a significant blow to the sternum that was pretty sore as well.
“It was probably an easy out for us, really. I’m not saying that lightly, but he did have an emotional week. We looked at it and that was possibly the deciding factor.”