The 22-year-old Thuram headed in at the far post from fellow Frenchman Plea's cross in the 41st minute after Florian Neuhaus had put the hosts ahead in the 17th.
Thuram celebrated by taking a knee in honour of ongoing protests in the United States following the death of George Floyd.
Sebastian Andersson briefly cut the deficit for the visitors five minutes after the restart but Thuram was again well placed at the far post when Plea threaded another pass through the Union defence on the hour.
Plea, who has 10 assists, turned scorer himself with a well-timed left-footed shot to join Thuram on 10 goals this season.
Plea, who has 10 assists, turned scorer himself with a well-timed left-footed shot to join Thuram on 10 goals this season and put Gladbach back in the mix for a Champions League spot with their first win in three games.
"We did what we set out to do," Gladbach coach Marco Rose said. "When we had possession we were dominant even though we made one or two mistakes more than we should. But we scored four good goals and I am obviously very satisfied with the performance and the result."