Gregg is an enigma.
The men loved him because he was modest and unpretentious. At the same time, he had all of the charisma of an old shoe, which means that he doesn't make for the most interesting subject for a biography.
Gregg is an enigma. He was capable of brilliant performances, such as his magnificent performance on East Cavalry Field at Gettysburg. Most of the credit for the victory there belongs to him. But other times, he seemed slow and stupid. His performance at Brandy Station on June 9, 1863, as an example, left a lot to be desired. He was slow that day, and left Buford hanging for hours. During the retreat from Gettysburg, he was missing an action, a complete non-factor. Some of that was due to Alf Pleasonton's failures to be an effective corps commander, but Gregg, in the absence of orders from Pleasonton, showed no initiative, which is why he was missing in action for most of the ten days after the Battle of Gettysburg.
It's unknown why he resigned his commission in February 1865. Dr. Alphonzo Rockwell, the regimental surgeon of the 6th Ohio Cavalry, wrote a memoir near the end of his life. This is what Rockwell said: "I was not a little surprised to hear Gregg say to [Brig. Gen. Charles] Smith that he was about to resign from the army. 'The fact of it is,' said Gregg, 'I am a good deal of a coward. Every engagement tells upon my nervous system to the last degree, and it is only the exercise of all my will power that I can appear natural and unafraid.'" There is absolutely no corroboration for this claim, which was made in 1920, 55 years after the fact. Gregg himself never said, and there is nothing else to substantiate it.
My theory is this: Gregg watched Phil Sheridan wreck the careers of his West Point classmates and friends, William Woods Averell and Alfred Torbert, and figured he was next. Sheridan had already hung Gregg out to dry at Samaria Church on June 24, 1864, when Gregg's two brigades were pounced upon by six brigades of Confederate cavalry and only escaped by the skin of their teeth. Gregg was nearly captured. I think that he made a choice that he didn't want to be next, and that he was unwilling to serve under Sheridan again after what Sheridan had done to him once and what he had done to Averell and Gregg, so he resigned his commission. I have absolutely no evidence in particular to support that theory, but knowing what I know of Gregg, it is a sound and reasonable explanation.
We do know that Gregg regretted his choice later. He tried to get a Regular Army commission after the war, but Sheridan would not allow it. Big surprise, right?