The X-Gw-Timestamp request header contains the UNIX epoch (UTC) in milliseconds, such as 1648816933000. Some implementations of strftime provide this as %Q. In Ruby for example:

require 'date'
timestamp = DateTime.now.strftime('%Q')
request['X-Gw-Timestamp'] = timestamp
var timestamp = (new Date).getTime().toString();

If this is unavailable in your language or framework, multiplying the epoch by 1000 will be adequate.