WWell, according to the Íslendingabók database, Guðmundur was the son of Ragnhildur Þorvarðardóttir who was living with him at Jökulsá in the 1801 census.She was born in 1719.That is all there is known about his ancestry or relatives.His father is unknown, as are his siblings and the encestry of Ragnhildur.
With Ólöf, the situation is a lot better.
Her father was Guðmundur Narfason, b. 1750, son of Narfi Hinriksson and Ingiríður Bjarnadóttir.
Guðmundur Narfason never married, but the mother of Ólöf Guðmundsdóttir was Ólöf Ormsdóttir, 1745-1834.She was a widow at the time - her first husband was Jón Þorvarðsson, and in 1794 she married Jón Guðmundsson.Her parents were Ormur Gissurarson and Vilborg Sigfúsdóttir.It is relatively easy to trace the ancestru of bóth Guðmundur Narfason and Ólöf Ormsdóttir back all the way to the settlement of Iceland in the 9th century.