It seems that the option to display the grade statistics for the assignment in New Gradebook is gone (the old "Assignment Details" option that displayed AVG, HIGH and LOW scores, and number of submitted assignments). I couldn't find any other way to display statistics in New Gradebook. The closest I was able to find was to click on the assignment title in the column header, then Speed Grader, but you can find there only AVG statistics. 

Does anyone knows any other quick way to view grade statistics for individual assignment?