I do not think there is any way to explicitly exclude tables from GSS. Workaround might be (although not an optimal solution) to run GSS on required schema/tables explicitly. Also you can create a script to include possible tables & run GSS from DB directly rather than using concurrent request.
Can you refer the below document and do find for load_xclud_tab
Best Practices for Minimizing Oracle E-Business Suite Release 12.2.n Upgrade Downtime (Doc ID 1581549.1)
Hope this helps!
You can lock the statistics by using DBMS_STATS.LOCK_SCHEMA_STATS.
GSS may throw a warning, but it should not overwrite the statistics.