Per HTTP Standards it is expected that either a content-length header will be present for responses which are under 8k in size or there should be a transfer-encoding header for responses which are greater than 8k in size. In gateway 10.x when dealing with a large response which should require a transfer-encoding header to be present, when the return template response
to requestor assertion has the 'Send Response Immediately' option enabled the transfer-encoding header is not present.
If not required, disable the 'Send Response Immediately' option from the Return Template Response To Requestor Assertion and the Transfer-Encoding header will then be present on responses which are greater than 8k in size.
We have an internal issue opened with our development team to further review this situation and get this addressed in a future CR.