Are you able to set those Status and HTTPheaders straight into the instance of EnsLib.HTTP.GenericMessage in your code. like below

Set httpGeneric = ##Class(EnsLib.HTTP.GenericMessage).%New() ; this was already initiated in your business host via code.
set att("Status")=..#HTTP400BADREQUEST
do httpGeneric.SetAttributes(.att)
set httpHeader("ContentType")="application/json"
set httpHeader("totalcount")=totalcount
do httpGeneric.SetHTTPHeaders(.httpHeader)

Hello @Enrico Parisi 

I just a generated text by executing this  set data="" for i=1:1:400 set data=data_"a"_i and it's 1492 characters long and it's not other JPEG or other file formats. But this text is not compressed. However, If I use random text/place holder text more than 1500 characters and that text was compressed automatically.

Awesome! Welcome back Global Masters and looks pretty good!