Downloading a portion of a Drive file in Apps Script.

downloadFileRange.gs

function downloadFileRange(fileId, startByte, endByte) {
  // Mention DriveApp in a comment to ensure the Drive scope is requested.
  // DriveApp.getRootFolder(); 
  var url = 'https://www.googleapis.com/drive/v3/files/' +
      fileId + '?alt=media';
  var response = UrlFetchApp.fetch(url, {
    headers: {
      Authorization: 'Bearer ' + ScriptApp.getOAuthToken(),
      Range: 'bytes=' + startByte + '-' + endByte
    }
  });
  // Assuming it's text content.
  return response.getContentText();
}