Gregg began his professional career with a regional accounting firm. He practices in the areas of taxation, trusts and estates, wills, probate and business law. Gregg represents clients in tax controversies before the Internal Revenue Service, the Kansas Department of Revenue and in the federal and state courts. His representative clients include individuals and closely held businesses.

He is admitted to practice in the U.S. District Court for the District of Kansas, the U.S. Tax Court and the Kansas state courts.

Gregg lectures on tax and related topics to professional and various other organizations. He is also a lecturer at Wichita State University. He is listed in The Best Lawyers in America for Wichita, KS, 2018 in the practice area of Trusts and Estates.