How do you get json from batch request?

I'm getting the following error if I console.log my fetch response.json():

Uncaught (in promise) SyntaxError: Unexpected number in JSON at position 3

If I dive into network sources to see the response (by the way it has status 200) in devTools I see the following line producing the error:

--batch_SuWHKrAohxj1o_r1qo6yzaAu-gfaqQ1p <-- producing error. Red lines underneath.
Content-Type: application/http
Content-ID: response-

HTTP/1.1 200 OK
Content-Type: application/json; charset=UTF-8
Vary: Origin
Vary: X-Origin
Vary: Referer

{
..........
..........
.........
}

My fetch() call has following headers:

  headers.append('Authorization', `Bearer ${token}`);
  headers.append('Host', 'www.googleapis.com');
  headers.append('Content-Type', 'multipart/mixed; boundary="deel_foo"');
  headers.append('Accept-Encoding', 'gzip');
  headers.append('Accept', 'application/json');

fetch(.....).then(response) => response.json())

each batch request inside body has Content-Type: application/json

EDIT:
I want the following response.

// stripped off the following:
--batch_K_aZ7aQTR91ApoeMUayvTEJhZBs6PW9n
Content-Type: application/http
Content-ID: response-

HTTP/1.1 200 OK
Content-Type: application/json; charset=UTF-8
Vary: Origin
Vary: X-Origin
Vary: Referer
// stripped off till here.
{   // I want this part, as it's valid JSON.
  "id": "17vegjeogjeogje44",
  "threadId": "17vjorgjerogjerogjer",
  "labelIds": [
    "CATEGORY_UPDATES",
    "INBOX"
  ],
  "snippet": "Don't miss this idea and many more. Stash what matters to you and inspire others. \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c \u200c",
  "payload": {
    "partId": "",
    "mimeType": "text/html",
    "filename": "",
    "headers": [
........
etc
........
etc........
}