The Peter Keriacos I posted about previously, in response to another person named Foster, has a WWI draft registration card which lists his father as a Gust Keriacos, residing in XXXXXX (possibly Korinthe) Greece.
That Peter Keriacos was born in the same town in Greece as his father, on March 14, 1897.
But Gust/Gus is a very common Greek first name.
If the following is your Peter, maybe we can find and use his naturalization certificate to get to his hometown and father in Greece:
1930 Indiana State census, North Twp, East Chicago city, Lake County:
Peter Keriacos, age 33, b. Greece. Wife May, b. Missouri, age 23. No children. Peter is a House painter; rents at 182 Fir Street. First arrived in US in 1914. Is a naturalized citizen of the US, but not a military veteran.
Peter Keriacos, age 43, b. March 14, 1877 in Greece, resides in a hotel/boarding house in Lake Twp, East Chicago City, Indiana. Single, laborer in steel mill. [Note: Someone later corrects age to 23, DOB to 1897.] First arrived in US in 1914 and currently has provisional application for US citizenship on file.
WWI Draft Reg. Card DOB March 14, 1897. Address is something like 3440 Pxxx Cxxxx, XXXXX, Hrbr, Ind. DOB March 14, 1897, XXXXXX (possibly Korinthe) Greece. Father Gust Keriacos, also of XXXXXX (possibly Korinthe) Greece. Peter works in "koke plant, Indiana Harbor Works". Arrived in US in 1914.
Looks like these 3 listings might all be for the same person. Let me know if any of it fits. If this is correct, you might be able to work from his citizenship papers back to Greece, knowing the correct date and place of birth, port of entry to US, etc.