Get a dozen guys to dress up like them and go stand around with them. When things start happening, just run around and make accusations and cause general mayhem.
How can you impersonate someone who has no markings, and no ID?
They literally stop you on the street and demand to see your papers. And then take you away to some gulag if they don't like what they find. If it walks like a duck and talks like a duck...
The Gestapo wore uniforms and could always be noticed To quote Major Hochstätter to Colonel Hogan, senior POW in Stalag 13: "The Gestapo is never friendly and never takes prisoners"