You have a wealth of resources to prove she exists even if you never find a birth/baptism record for her.
You know her maiden name is Duncaton/Dunkerton - source Marriage record.
The baptisms records/sources on familysearch for George and Mary's children will also confirm relationships and existance - list of children John b1814, Hannah b1826, Jane b1828, Mary Ann b1832 and Sarah b1835. (excludes William b.c1818 and Daniel b.c1823 - could not locate a baptism record for these two). The records show this family to be the only George and Mary Garland in Bedminster at that time.
You have her approx birth year of 1797 and birthplace from the 1841 census (plus or minus 2 to 3 years). source: "England and Wales Census, 1841," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:M7MZ-SK7 : 13 December 2017), Mary Garland in household of Joseph Starrs, Bedminster, Gloucestershire, England; from "1841 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing PRO HO 107, The National Archives, Kew, Surrey.
Her death registration in 1849, - source: - FreeBMD, https://www.freebmd.org.uk, "Deaths Dec 1849 GARLAND Mary Bedminster 1149.". Approx birth year is further supported by her age on her burial records. source: "England, Bristol Parish Registers, 1538-1900", database, FamilySearch (https://familysearch.org/ark:/61903/1:1:XTST-GW1 : 10 March 2018), Mary Garland, 1849.
In case you do not have - George's re-marriage in 1850 source material
Marriage of George Garland on 9 Sep 1850 at St John, the baptist, Bedminster to Mary Harding - spinster. George was 59 years old, widowed, living at North Street, fathers name is William Garland, Coal miner Sources: FreeBMD, https://www.freebmd.org.uk, "Marriages Sep 1850, Garland George Bedminster 11 79, Harding Mary Bedminster 11 79.", "England Marriages, 1538–1973 ," database, FamilySearch (https://familysearch.org/ark:/61903/1:1:NNXP-V1H : 10 February 2018), George Garland and Mary Harding, 09 Sep 1850; citing Bedminster, Somerset, England, reference p111 item222, index based upon data collected by the Genealogical Society of Utah, Salt Lake City; FHL microfilm 1,849,435., Somerset Marriage Index, Somerset Archives, Parish Marriages, Archive ref P/St.JBed/R/3/15, viewed on findmypast.co.uk.
And his siblings - List of births and baptisms dates of George and his siblings as listed in Index's (all baptised at St John the Baptist, Bedminster, Somerset). source: refer to familysearch.
John b 21 Nov 1794 bapt 27 Jul 1806
Amey b 20 Feb 1795 bapt 4 Jun 1797
George b 31 Jan 1797* bapt 27 Jul 1806
William b 11 Apr 1797* Bapt 4 Jun 1797
Sarah b 24 Jul 1801 bapt 5 Nov 1801
Daniel b 9 Jul 1804 bapt 23 Sept 1804
Phoebe b 2 Jul 1806 bapt 27 Jul 1806
William b 9 Nov 1809 bapt 4 Jun 1809
Isaac bapt 27 Jan 1813
The parents - William Garland and Sarah Matthews were married by banns on 16 May 1790 at St John the Baptist in Bedminster. Witnesses Samuel Cloud and Benjn Price. Source: Somerset Marriage Index, Somerset Archives, Parish Marriages, Archive ref P/St Jbed/R/3/s, viewed on findmypast.co.uk.
Also note that there is a discrepancy in the birth dates for George and William. For George, I suspect there may be a transcription error with his birth date. George's 1st marriage in 1813 would have at around 15-16 if he was born in 1797. Other records point to a birth year of around 1791. Suggestion is to validate against original records to confirm birth year.
I hope this helps