I ran "execute DBMS_STATS.GATHER_SCHEMA_STATS('my_schema_name',100);" and all I saw for the output was "PL/SQL procedure successfully completed". Where can I see the actual stats gathered by this procedure?
I was expecting to see the schema stats gathered by this stored procedure, but I just saw text confirming that it completed successfully.
dbms_stats.gather_schema_stats gathers statistics on all stale table and index segments in the entire schema, so the amount of information would be impossible to digest if it were to display it. Further, the purpose of gathering stats is not for human consumption, but to arm the optimizer with the metadata it needs to make reasonably good predictions about cardinalities (rowcounts) coming from object access and joins so it can develop good execution plans. It's for Oracle's use, primarily, not ours.
Having said that, manually inspecting statistics can be useful in debugging bad execution plan decisions the optimizer might be making for some poorly performing SQL you're trying to tune. In that case, you can see that by querying various views:
dba_tab_statistics will show table-level and partition and sub-partition table statistics.
dba_tab_col_statistics will show column-level statistics
dba_tab_histograms will show histogram buckets that may have been gathered for some columns.
dba_ind_statistics will show index and partition and sub-partition index statistics.
Some of this data is also joined into and exposed within the various segment-level object views if you prefer to see it that way (dba_tables, dba_tab_partitions, dba_tab_subpartitions, dba_indexes, dba_ind_partitions, dba_ind_subpartitions, dba_columns/dba_tab_cols, etc..)