Final answer:
Gerda's family members were likely unreachable because the Nazis had rounded them up and taken them from their homes as part of the Holocaust. They could have been in hiding, sent to ghettos, concentration camps, or extermination camps, thus disrupting communication.
Step-by-step explanation:
On the day of the Nazi invasion mentioned in the question, there could be various reasons why there were no replies when Gerda attempted to telephone other family members. During the German occupation and invasion of various countries in Europe, many Jews were forcibly evacuated and relocated by the Nazis.
They were sent to ghettos, concentration camps, or extermination camps as part of the Holocaust. The explicit answer to where these people have gone is not provided in the text, but historically, it is likely that Gerda's relatives were unable to answer because they had been rounded up by the Nazis and taken from their homes.