fetch('./data.json.gz',{"Content-Encoding": "gzip" , "Content-Type": "application/json"})
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(err => console.log(err));

result:

SyntaxError: The string did not match the expected pattern.

data.json:

{
  "data":1
}

and it was gzipped with gzip data.json

According to answer here this should work?
How to fetch json.gz in a modern browser without third-party libraries?

Is this possible – are my headers insufficient?