Hi Leandro,
We have provided plugin source with the attached Sample, you can modify App42GCMService.java file according to your requirements and enhance Notification UI.
You can build the jar file with the plugin source and replace the same in Android folder of your Unity project.
By default plugin shows the application icon name as app_icon from drawable folder.
Let me know if you face any issue in customization.
Thanks
Vishnu Garg