GSM-45946
Source database and target db shouldn't be the replica leader for the given replication unit. If target db already contains this replication unit it will be replaced by full replica of replication unit on source db.
If -source is not specified, then an existing follower of the replication unit will be chosen as the source database.
If -source is not specified, then an existing follower of the replication unit will be chosen as the source database.
Cause
GDSCTL help message.