Well said. It's important to add that he was not just a Nazi trying to weasel his way out of punishment at the end of the war, though. He worked covertly and overtly against the Nazis for many, many years, even before he joined the SS.
Here's some more facts about his involvement with the National Socialist Party:
In early 1935 he stood up in a theater during a performance of the play Wittekind protesting against its anti-Christian message and was beaten up by Nazi Party members in the audience.[12]
He also came into conflict with the Nazi government for distributing anti-Nazi material. He was arrested for the first time on 4 September 1936, held in protective custody for five weeks, and expelled from the Nazi Party.
- [He had a] plan to join the SS "to see things from the inside", to try to change the direction of policies, and to publicise the crimes being committed.[15] Browning describes him as "a covert anti-Nazi who infiltrated the SS ..."[16] And in a letter to his wife he explains: "I joined the SS ... acting as an agent of the Confessing Church."[17]
He was definitely complicit in the gas chambers:
Because of his technical education, Gerstein rose quickly to become Head of Technical Disinfection Services...He supplied hydrogen cyanide (Zyklon B) to Rudolf Höss in Auschwitz
In 1942, several days after witnessing "the gassing of some 3,000 Jews who had arrived by train from Lwow," Gerstein "had a chance encounter on the Warsaw to Berlin train with Swedish diplomat Göran von Otter, who was based in Berlin. During a conversation which lasted several hours, he told the Swede what he had seen and urged him to spread the information internationally.[20]"
As well,
- He attempted to make contact with representatives of the Vatican, the press attaché at the Swiss Legation in Berlin, and a number of people linked to the Confessing Church.[21]
Despite trying until the end of the war, the Vatican ignored his messages.
Though he's been criticized by critics such as Historian Christopher Browning for exaggerations, even he wrote this:
But in the essential issue, namely that he was in Belzec and witnessed the gassing of a transport of Jews from Lwow, his testimony is fully corroborated .... It is also corroborated by other categories of witnesses from Belzec."[16]
Finally, regarding your assertion that "it was evident he was going to be executed," there's this quote from Wikipedia in the section regarding his arrest and death:
On 22 April 1945, Gerstein surrendered to the French commandant of the occupied town of Reutlingen. He received a sympathetic reception and was transferred to a residence in a hotel in Rottweil.
I don't think they would have put people they are planning to execute in a hotel. They usually go to a prison or at least a brig.