Gap resolution of Logical Standby Database via Archive Log Backups
If you can see the backup file in the directory, try cataloging it:

RMAN&gt; catalog starts with '/path/to/archive/directory';


It will list the backup and ask you if you want to catalog it.
Following which you should be able to continue with your gap recovery:

alter database recover managed standby database cancel;
recover standby database;
#after all existing logs are applied put standby database to recover managed 
mode
recover managed standby database


If you don't have the archived logs anymore, you can do an incremental backup with:

"backup incremental &lt;from scn&gt; until scn"