Use V$ARCHIVE_GAP from standby database
1) How to find archive log gap from Primary Database site?
DB_UNIQUE_NAME is used to distinguish between primary and standby database and it is also helpful when you configure Dataguard broker.
2) Do I need the DB_UNIQUE_NAME when setting the LOG_ARCHIVE_DEST_2 ? What is the purpose of DB_UNIQUE_NAME in LOG_ARCHIVE_DEST parameter?
In your query SYNC Means there is difference in archive sequence of primary and standby database.
3) If archivelog gap happens standby db goes out of sync with the primary database. What does out of sync actually mean?
4) Primary DB knows where to transport redo data based on the location mentioned in LOG_ARCHIVE_DEST_n of Primary DB. Am I correct in my understanding?