Dear Mr. Phillips,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the series Reports of Site Locations, 1837 - 1950 in the Records of the Post Office Department (Record Group 28) that includes a file unit for Georgia: Candler - Chattooga which contains records for the Graysville P.O. beginning on image 358. We also located a series titled Records of Appointment of Postmasters and the Establishment of Post Offices in the Records of the Post Office Department (Record Group 28) that contains records for Catoosa County Georgia. These records have been digitized and are available using the Catalog.
We hope this is helpful. Best of luck with your research!