If you are talking about SMA > Logging and Tracing > Garbage Collector and Log Usage feature, then it has been removed from Pega 7.3 onwards, refer BUG-285148. If you are looking for the GC file storage location then probably you would like to check in your application server configuration regarding the gc jvm parameters.
Is there any documentation to set some run time variable configuration file so that GC log can be obtained. We tried below PDN post. But still we could not find link designer studio Pega Designer Studio Lin k==> System ==> Operation ==> Logs
As per the suggestion provided by my colleague Ujjwal on referenced post the logs will be stored in tomcat_location/logs/gc.log file and i think you need to raise cloud support request to get those logs if your environment is running in Pega cloud and don't have access to this folder from Self Service Portal. If it was on-premises then you could probably look into the tomcat/logs folder to get the gc.log file. Hope that answers your query.