Forgive me if I suggest things you've already tried, but here are a few suggestions for possibly finding Gordon's parents.
Have you ordered Gordon's death certificate yet?That should tell you the names of his parents, to the best knowledge of the informant.
You may want to check the 1880 everyname census index, available for free at www.familysearch.org.You may be able to narrow down your Gordon with his parents.
It's always a good idea to find this couple in the other censuses, too.Gordon's parents or a sibling may be living with them.I don't have access to the 1900 or 1910 census indices, and I couldn't find Gordon in a quick search of the 1920 census.Here is his entry in 1930:
1930 Missouri, Dunklin County, Union, ED 31, Page 6B
Dwelling 106/Family 106 owned farm
Gordon Miller, head, white, 57, married, age 23 at first marriage, reads, writes, b KY, father b United States, mother b KY
Mirtie, wife, white, 53, married, age 20 at first marriage, reads, writes, b Missouri, father b Tennessee, mother b Indiana
Paul, son, white, 18, reads, writes, b Missouri
Alva, son, white, 17, attends school, b MO
Eunice, daughter, white, 13, attends school, b MO