Some dates of birth and death even if only approximate for the children would have helped. The name of Gottlieb's wife would have too.
The 1900 census of Reserve Twp., Allegheny County, PA has this family:
Gottlieb Fischer,born Dec 1845 in Germany, married 19 years Katherine A., born Oct 1849 in Germany J. Fremont, born April 1882 Chas H., born Oct 1886 Edwin M., born Oct 1887 George G. M., born Dec 1888 Harrison R., born Aug 1892 Albert G., born Sep 1894
The 1910 census shows the family living in Pittsburgh. By this time Gottlieb is a widower. He probably died by 1920. J. Fremont becomes John C. F.
You can look at these records for free at beta.familysearch.org.
If you know where and when Gottlieb died I suggest getting his death certificate and obituary. If Katherine died after 1905 the PA Dept of Health would have her death certificate. If she died before 1906 he death may have been recorded at the Register of Wills in the county she died in. However, unlike a death certificate it will probably not have her parents names. It will however indicate where she is buried and undoubtedly Gottlieb is buried with her.
If you don't know the death date for either you could check with the cemetery they are buried in if you know which one. There is a chance an estate was filed for either one at the Allegheny County Register of Wills. A series of books were compiled on the burials in Allegheny County. I believe the State Library in Harrisburg has copies as does the Carnegie Library in Pittsburgh. Undoubtedly others in the area do as well.
The death certificates and obits for at least one of their children should have Katherine's maiden name.
Check with the Western Pennsylvania Historical Society and the Historical Society of Western Pennsylvania to see if they can help you.