it also may be that I was lucky that they are (partly) gifted with math (like me).

Peer reviewed papers suggesting math skills are inherited:

• The Genetic and Environmental Etiology of High Math Performance in 10-Year-Old Twins
• The Heritability of Aptitude and Exceptional Talent Across Different Domains in Adolescents and Young Adults

Peer reviewed paper suggesting how parents discuss math influences child math skills:

• Some types of parent number talk count more than others: Relations between parents’ input and children’s cardinal-number knowledge

Jean Piaget had so many American parents ask him how to accelerate their child's development that he came to call this "the American question." I do not think Gunnar is an American, but I believe it is the American question being asked here. I do not know Gunnar or his children, and the only general statement I can make about them is it appears he is a good parent and has clever children.