Prior to 1933, there was the Prussian Secret Police, "Abteilung I A". As such, it was a police unit, i.e. working there was very much voluntary.
In 1933, the Prussian Secret Police was renamed "Geheime Staatspolizei" (Secret State Police), abbreviated Gestapo. (Which made all the staff of the Prussian Secret Police -- volunteers -- now "Gestapo members".)
Legal restrictions on their actions were removed step by step, and the organization eventually separated from the police apparatus and placed under direct control of Göring (Secretary of the Interior) and later Himmler (Chief of German Police and Reichsführer of the SS) .
At no point did the Gestapo press people into working there. I can imagine that quitting service with the Gestapo, or turning down a transfer, could raise some eyebrows. But it was a voluntary organization (as far as anything under totalitarian rule can be voluntary).
The beginnings of the Schutzstaffel in 1925 were as personal bodyguards and thugs to Adolf Hitler (very much voluntary). The organization grew and became more militarized, but well into the times of the Waffen-SS it remained a voluntary organization.
Only from 1943 onward (or possibly as early as February 1942) did the Waffen-SS draft men into service.