Tying all together.
The gelf.xml should be placed in app/config and included in your config.yml:
imports:
- { resource: gelf.xml }
Adjust the parameters to your needs (parameters.yml/parameters.ini):
parameters:
graylog.hostname: graylog.example.com
graylog.port: 12201
# DEBUG
# monolog.handler.gelf.debug_level: 100
# INFO (default)
monolog.handler.gelf.debug_level: 200
# WARNING
# monolog.handler.gelf.debug_level: 300
# ERROR
# monolog.handler.gelf.debug_level: 400
# CRITICAL
# monolog.handler.gelf.debug_level: 500
# ALERT
# monolog.handler.gelf.debug_level: 550