Well the only Garner, that was an officer, that served between 1910 and 1920, in the database at Ancestry: "U.S. Marine Corps Muster Rolls1798-1940" and Ancestry's source cited is: Muster Rolls of the U.S. Marine Corps, 1798-1892; (National Archives Microfilm Publication T1118, 123 rolls); Records of the U.S. Marine Corps, Record Group 127; National Archives, Washington, D.C. U.S. Marine Corps Muster Rolls, 1893-1940; (National Archives Microfilm Publication T977, 460 rolls); Records of the U.S. Marine Corps, Record Group 127; National Archives, Washington, D.C., is:
John L Garner Jr, first muster entry is April 1917 as Second Lieutenant in "National Naval Volunteers, Marine Corps, 36th Company Marine Barracks Mare Island, California" under remarks is noted "NM". At bottom of pages listing all officers and enlisted men it is noted..."all members of this company enrolled at Los Angeles, California April 6, 1917 unless otherwise stated in column of remarks".
November 1917 he is listed as a First Lieutenant at Marine Barracks Quantico, VA remarks: "1-30 under instr"
Sept 1918 he is listed as Captain, Company A Officer's Training Camp Marine Barracks Quantico, VARemarks; 13 aptd Capt to rank to July 1st auth MCO #46 Series 1918.
Last Muster entry is dated June 1919 as Captain Marine Barracks Navy Yard Mare Island California. 1st Marine Corps Reserve Class 1 Bunder Detached heading and Captains- John L. Garner Jr. Remarks 1 to 24 in charge of Machine Gun School Post 18. Requested discharge by wire. 25. Detached and ordered home to wait discharge.
There are other entries dated between what I already posted but you can look it up yourself, check to see if your local library has a subscription to Ancestry or use an LDS Family History Center to rent NARA microfilms, or go to an NARA library.