The below code will delete all .tar.gz files which are older than 240 days.
find /backup/cpbackup/ -mtime +240 -name "*tar.gz" -exec rm {} \;

If you are doing any modifications in the above code then make sure that before executing the delete command you do a dry run with ls command and get the desired find results
find /backup/cpbackup/ -mtime +240 -name "*tar.gz" -exec ls {} \;

-exec passes the search results to the succeeding command.
Place the code in a .sh or just a text file and make it executable by
chmod 644 file_name.txt
Now set a cron to execute this file every 3 months or so.