Could you try it with TCP port? For that, edit your
/opt/bitnami/logstash/conf/logstash.conf file, and set the following gelf parameters.
use_tcp => true
host => "0.0.0.0"
port_udp => 12201
port_tcp => 12202
After that, restart logstash service:
sudo /opt/bitnami/ctlscript.sh restart logstash
Anyway, I suggest you consulting GELF documentation, and Elastic support site, where they can provide you with better information about the plugin configuration and usage.
I hope it helps