Gustav is een zoon van Isaak Müller en Lea Meijerhoff. Hij trouwde 19 juni 1913 met Regina Druijf (20 jaar), dochter van Heiman Druijf en Klaartje van der Ziel.
The Leiden police commissioner requested that Gustav Müller, a shopkeeper, and his children Edith and Hans, residing in Leiden, be located detained and brought to trial. They were suspected of having changed their place of residence without the required authorization. Requests that Gustav Müller's wife be located were issued as well. Algemeen Politieblad, nr 40, 8 October 1942, 1136, notice 2192