What was the “turning point” in the showdown when it became apparent that Gatsby was not going to “win” this confrontation? Explain.

The turning point occurs during Gatsby's confrontation with Tom, the moment Daisy can't deny that she did love Tom once.

Explanation:

It is this moment in which it becomes apparent that Gatsby is not going to "win" the confrontation because his hopes to erase the last 5 years can't be maintained. It is impossible for Daisy to deny her past and to not accept it. It is here when we realize Gatsby has asked for too much and will not win.