The German historian Hans Rothfels writes about Gerstein;
https://muse.jhu.edu/article/846188/pdf
"The question remains, however, whether it was even possible for
G. to make observations on the scale he maintains and whether these observa-
tions are internally credible."
"The results of this credibility test can be summarized as follows: the available evidence completely
confirms the core of the statements (religious propaganda, twice in protective
custody, dismissal from the party) and the early party membership (May 1933) is
willingly admitted, but G. tends to overtax his memory when it comes to the accu-
racy of certain details (e.g. 2 October instead of 15 October; 14 October instead of
23 July)."
" Was it even logistically possible (200 Ukrainians, approx. 100 chairs,
12-13 lashes)? Is it possible to remember such specifics? It must be clear that not
every word can be taken at face value and that it is entirely possible that there
are mistakes or inaccuracies in terms of the incidentals..."
Rothfels finds that Gerstein is not that credible, he makes mistakes, makes claims that are not possible, but his core claims are corroborated. He disputes that claim by the Bishop of Berlin that Gerstein's claims were "absolutely true". Instead, he states;
" The historian, bound by his methods, cannot go quite so far in his conclu-
sions, but indisputably, this witness and his verification of the matter weigh
heavily alongside the other corroborating evidence."
Here is another example of a historian, who has assessed Gerstein as unreliable and provided examples of why that is. He then applies the test of corroboration and finds that the primary claims about gassing are verified by other evidence. As for PS-1553, Rothfels states;
"A number of the references provided in the PS-1553 file and in the German version have been questioned or
testified to in the Frankfurt trial."
"The question remains, however, whether it was even possible for
G. to make observations on the scale he maintains..."
Gerstein's version of events has been questioned and subjected to extensive checking and verification. He passes the primary test of corroboration, but he fails many of the tests regarding the accuracy and credibility of his claims.
Sanity Check - "Thus, currently revisionists can console themselves by affirming their incredulity..."