Did you read GELF specification correctly?
Timestamp parameter should be unix epoch timestamp, and not date string:
https://docs.graylog.org/en/3.3/pages/gelf.html#gelf-payload-specification
-
timestamp
number -
Seconds since UNIX epoch with optional decimal places for milliseconds; SHOULD be set by client library. Will be set to the current timestamp (now) by the server if absent.