Timeline

05/08 SEP 1853 Gerrit Tjerks Wassenaar was born in the village of Sint JacobiParochie
30 SEP 1876 Bought a house and windmill in the village of Witmarssum
DEC 1876 Leaving from Sint JacobiParochie, occupation "koopman"
08 DEC 1876 He's signing a rental agreement, together with Dirk Visser, to rent property from Abel Wiarda and Eeltjes Kooistra
15 DEC 1876 He had a loan for fl.12500 from Johan Winkler, living in Haarlem ( relation is unknown )
15 DEC 1876 He had a loan for fl.8800 from Aconia Holwerda, living in Groningen ( relation is unknown )
07 MAY 1877 He had a loan for fl.2500 from Minke Klazes Dijkstra
07 JUL 1877 He had a loan for fl.6000 from his familiy in law, Roelof Schuling from Sint Jacobiparochie.
1900-1920 According to municipality records he's born at 08 SEP 1853 and has no religion, occupation is named "werkman" living in Sint Jacobiparochie nr141 26 AUG 1919 Passed away, according to records in Leeuwarden.
16 DEC 1919 according to a sort of testimony ( since it's after his date of death ) living in Sint Jacobiparochie

At some moment he's named "ambteloos burger".

Windmill

From the timeline we see Gerrit T Wassenaar in 1876 is loaning a large sum of money from multiple people, to buy this windmill in Witmarsum.

Some more info can be found on the windmill, in 1859, a man named Reinder Hamming buys the windmill from Teetske van der Wal. Reinder manages to hold it until 1872 after when he becomes a shopowner in Balk and sels the property to Johannes Klazes Ypey, meesterbakker in Stiens.

Jehannes sels the property four years later in 1876 to Gerrit Wassenaar, who at that time is 22 and comming from Sint Jacobiparochie.

Jehannes Ypey keeps living in Witmarsum and continues to trade in seeds and nuts, Gerrit however struggles financially and just 2 years later in 1878 the mill is sold to a new owner.

Most likele to a Pieter Bosma 26 jaar and his wife Riemke Boersma