Your grandfather should be found in the record set U.S., Army Transport Service, Passenger Lists, 1910-1939 on Ancestry.com. You need a paid subscription, to sign up for a free trial, or to gain access through a local library using your library card (check to see if your library offers this).
Make sure to look for two entries--an outgoing list (to France) and an incoming list (back to the U.S.).
thanks, I will give them a try.
Do you have any additional information on your grandfather? Name, DOB, and place of birth?
Dear Mr. Barr,
Thank you for posting your request on History Hub!
For your information, the Ancestry data collection U.S., Army Transport Service, Passenger Lists, 1910-1939 referenced by Ms. Yuhasz uses records from the National Archives record series Lists of Outgoing Passengers, 1917 - 1938 and Lists of Incoming Passengers, 1917 - 1938 in the Records of the Office of the Quartermaster General (Record Group 92). These records were digitized by Ancestry as part of a partnership between Ancestry and the National Archives. While the National Archives will eventually get copies of the digitized records to make available for free in the National Archives Catalog, at present the copies on Ancestry as well as their subsidiary Fold3 are the only copies available online. Under normal circumstances, you would be able to access both Ancestry and Fold3 for free at any National Archives location. However, due to the coronavirus public health emergency, all National Archives research rooms and presidential libraries are closed until further notice. Your local public, state, and university libraries may have institutional subscriptions to either or both websites and offer free access to their patrons.
We hope this is helpful. Best of luck with your family research!