I was unable to find any reference to him and Mary Magdaline, the female listed as his daughter in this profile. I did find other children with that middle name, but that doesn't hold evidence. The evidence that would match is that the children were all christened in Caroll County, Maryland, which would match with where Peter was buried.
Peter passed away and was buried in the United Church of Christ cemetery in Manchester, Carroll County, Maryland on 2 July 1859.
For further perusal here are the children I did find. Please remove these research notes if they turn out to be inaccurate. If these were his children, then it is highly possible he married a Mary Sense and not a Polly Sellers as appears evident in a user submitted profile on familysearch.org.
↑ "Maryland Births and Christenings, 1650-1995," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:F489-9PK : 12 December 2014), Peter Gettier in entry for Barbara Ellen Eve Gettier, 15 Nov 1831; citing Manchester, Baltimore, Maryland; FHL microfilm 941,099.
↑ "Maryland Births and Christenings, 1650-1995," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:F48Z-F22 : 12 December 2014), Peter K. Gettier in entry for Rebecca Ann Magdalene Gettier, 03 Sep 1834; citing ; FHL microfilm 13,200.