There's no way to know the age of this man/men or to learn whether he/they are the Christian Gerner named on Fred Gerner's death certificate as his father. Nonetheless, it's interesting to note what I've found. Names include Christ., C., Ch., Christian, and Christopher. Occupations include lock maker, laborer, saw maker, gent[leman], and locksmith. They may or may not be the same man.
My grateful thanks go to Claudia of Claudia's Genealogy Blog for suggesting I look at Historic Pittsburgh where I found more Pittsburgh City Directories than I needed (at least for this search).
1857, p. 72 Christian Gerner, lock maker, 39 Gibbon bt Chestnut and Magee
1858-59, p. 80 Christian Gerner, lab[orer], Penn'a av n Price
1861-62, p. 108 Christopher Gerner, B. H., 237 Penna av
1864-65, p. 110 C. Gerner, saw maker, Pa av n Madison
1869-70, p. 169 Ch. Gerner, gent, Fifth av
1870, p. 183 Christ. Gerner, Fifth av and Soho
1873-74, p. 214 Christopher Gerner, lab[orer], Fourth av, Soho
1876-77, p. 235 Christopher Gerner, 722 Fifth av
1877-78, p. 230 Christian Gerner, locksmith, 720 1/2 Fifth av
1878-79, p. 248 Christian Gerner, r 720 Fifth av, Soho
In the 1870 and 1876-77 directories, Gotlieb Gerner is named. In the 1873-74, G., John, and Robert Gerner are named. Relatives?
An obvious question: Where is "Chas. Gerner" who was a 19-year-old apprentice in the 1870 census? City directories usually list all males who were working outside the home.