LogbackConfigurer.java

import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.spi.ILoggingEvent;
import com.yammer.dropwizard.ConfiguredBundle;
import com.yammer.dropwizard.config.Bootstrap;
import com.yammer.dropwizard.config.Environment;
import com.yammer.dropwizard.logging.AsyncAppender;
import me.moocar.logbackgelf.GelfAppender;
import org.slf4j.LoggerFactory;

import java.util.HashMap;

public class LogbackConfigurer implements ConfiguredBundle<YourDropwizardConfiguration>{

    @Override
    public void run(YourDropwizardConfiguration configuration, Environment environment) throws Exception {
        final Logger root = (Logger) LoggerFactory.getLogger(org.slf4j.Logger.ROOT_LOGGER_NAME);
        LoggerContext loggerContext = root.getLoggerContext();
        GelfAppender<ILoggingEvent> appender = createAppender(loggerContext);
        // configure logger for the relevant package, and wrap the appender using async decorator
        loggerContext.getLogger("com.your.package").addAppender(AsyncAppender.wrap(appender));
    }

    @Override
    public void initialize(Bootstrap<?> bootstrap) {
        //do nothing
    }

    private GelfAppender<ILoggingEvent> createAppender(LoggerContext loggerContext) {
        GelfAppender<ILoggingEvent> appender = new GelfAppender<ILoggingEvent>();
        appender.setContext(loggerContext);
        appender.setName("GELF");
        appender.setFacility("logback-gelf");
        appender.setGraylog2ServerHost("your-graylog-host-name");
        // GELF lets you add custom fields - you need to configure them here 
        final HashMap<String, String> additionalFields = new HashMap<String, String>();
        additionalFields.put("clientIdentifier", "_client_id");
        appender.setAdditionalFields(additionalFields);
        appender.start();
        return appender;
    }

}