This was a bit confusing live, so we'll work backwards and explain what happened. It was not an inbounds violation (technically). Rather, David Stockton and Elias Harris had a brain fart.
This GIF is the best we could do, but there's context:
You can kind of see it from the other angle:
And now for what happened. Wichita State hit the second free throw, and it was Gonzaga ball. Stockton came back for the ball, thinking he would pass it inbounds. The problem? Elias Harris inbounded the ball to Stockton (Harris is out of frame in the first GIF) and Stockton flat walked out of bounds.
It was a lapse in concentration that gave Wichita State the ball back, and an error that Stockton, Harris and Gonzaga are going to regret. Stockton simply didn't realize Harris inbounded the ball, and got caught -- kind of his fault, but kind of not.
Wichita State hit a three after getting the ball back, as well, compounding the error and giving the Shockers a four-point lead.