No, that's not true. I've heard of the GDR being referred to as socialist plenty of times in Germany. The main party was even called the Sozialistische Einheitspartei Deutschlands (Socialist Unity Party of Germany).

> most people would think

This is not the case at all here in Germany.

As someone growing up in the east german (I was 15 in 1989), I can tell for sure that we've learnt in school that the GDR is an socialist country on its way to the communism.