How to develop changes on an Ansible Galaxy package

I was using an ansible galaxy module in an Ansible role.

group default vars in roles

Is it possible to have group vars defined in roles?

Ansible Galaxy: OpenJDK 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future

I am new to ansible and I am trying out this ansible galaxy role by geerlingguy:
https://github.com/geerlingguy/ansible-role-logstash