application.gam is a GeneXus Access Manager (GAM) configuration file which contains the Application Id of the GAM WEB Application of the KB, specified in Application ID property.
During prototyping time, it is transferred automatically to WEB-INF folder in case of java applications, and to the virtual directory in case of NET and Ruby applications.
When taking the application to production, you need to include this file in the deployment. See GAM - Applications deployment.
To get this value at runtime use the GetGUID method from the GAMApplication data type.
&GUID = GAMApplication.GetGUID()