He married Hermien van der Weide (also from the Province of Overijssel) in 1963. As "Gert and Hermien" they were a very popular musical duo for many years.
Timmerman and his wife became Christians in 1974. After their conversion they started a Christian musical career but later on they returned to their previous career.
They divorced in 1999. He remarried another woman in 2002. Hermien died in 2003.