Gestapo vs secret police

Gestapo and secret police both are nouns.

Word	Noun	Adjective	Verb	Adverb
Gestapo	Yes	No	No	No
secret police	Yes	No	No	No

As nouns, secret police is a hypernym of Gestapo; that is, secret police is a word with a broader meaning than Gestapo:

Gestapo: the secret state police in Nazi Germany; known for its terrorist methods
secret police: a police force that operates in secrecy (usually against persons suspected of treason or sedition)

Gestapo (noun)	secret police (noun)
the secret state police in Nazi Germany; known for its terrorist methods	a police force that operates in secrecy (usually against persons suspected of treason or sedition)