Last summer I wrote a php-gelf library with multiple goals:

• Possibility to use as standalone PSR-3 logger
• Higher flexibility for transports besides UDP
• Test-coverage
• PSR[0-3] compatibility

The result can be found at https://github.com/bzikarsky/gelf-php and is based upon f @lennartkoopmann's original Graylog2/gelf-php.

I'd be happy to also provide an updated GelfHandler/GelfFormatter for monolog if there is interest/use for it. It would be a one-time dependency break since IMessagePublisher and Message don't exist in the updated version.