Problem with @type concat plugin

[wiktor2200]

Hello, I'm trying to run @concat fluentd plugin which is used to combine multi-line error messages in Kubernetes cluster on GCP

Concat plugin official repo
I've tried sample code from repo documentation:

<filter **>
  @type concat
  key message
  multiline_end_regexp /\n$/
</filter>

When I applying those lines to configmap, pods gets CrashLoopBackOff and in fluentd logs there is error:

[error]: config error file="/etc/google-fluentd/google-fluentd.conf" error_class=Fluent::ConfigError error="Unknown filter plugin 'concat'. Run 'gem search -rd fluent-plugin' to find plugins" 

How to make this plugin works with fluentd on GKE? Is there any possibility to install it with gem (while spawning fluentd pods)?