The 1900 census shows John Gatens, born Nov 1859 in PA. His parents were born in Ireland. They were married 17 years. Susan is shown as born Sept 1863 in Scotland and immigrated in 1867. Daughter Katie was born July 1887.
The 1910 census shows much the same data except Susan immigrated in 1871.
The 1880 census shows two John Gatens born around 1859 living in Schuylkill County and their parents were born in Ireland.
Some of these records you can look at for free at beta.familysearch.org and Ancestry.com. For those you cannot see for free I suggest checking with your local libraries to see if they have Ancestry or Heritage Quest.
The index of Schuylkill County marriages includes Edward Cox and Catharine Gatens 16-1629. The Schuylkill County Archives should have the record.
I suggest getting John and Susan's death certificates from the PA Dept of Health. They should have the parents' names and of course Susan's maiden name. The death certificates and obituaries of at least one of their children should have her maiden name too.
Check the newspaper microfilms at the Tamaqua Public Library for the obituaries. However unless you know the exact date it can turn into a long search. Their obits may have also appeared in the Allentown newspapers as well especially Susan's. They are available on microfilm at the Allentown Public Library and the Lehigh County Historical Society.
The church they were members of may have something in their records that is helpful as might the cemetery they are buried in.
The Schuylkill County Historical Society has a nice selection of records. The County Archives has several records that might be helpful particularly from the Register of Wills and Recorder of Deeds Offices.