The (defunct) Val Biro Official Fan web page has a brief interview with Val Biro on this subject, adding to the wording found in Gumdrop Makes a Start.
HOW GUMDROP GOT HIS NAME
He set the levers on the steering wheel and pulled the choke out. Then
he pressed his foot on the starter button. The car went "Guuuuum". The
foreman pushed in the choke and pulled out the ignition button and
pressed his foot again. This time the car went "Drrrropopopop" and the
engine started.
The boy was enchanted. What a lovely sound; "Guuuuum-Drrrropopopop".
As if the car had called out his own name. "That's it!" he declared to
one and all. "That's his name. GUMDROP!" and he felt convinced that
Gumdrop was the only car for him.
This presumably is also the out-of-universe reason that Balint Stephen "Val" Biro named his real-world car (on which the book series is based).