On error, the applier thread writes the RBR buffer to a log file named GRA_xxx.log. This file can later be used during investigation.
The file, however, does not contain the binlog header, so in order to inspect the dump using mysqlbinlog, one first needs prepend it with binlog header.

http://www.percona.com/blog/2012/12/19/percona-xtradb-cluster-pxc-what-about-gra_-log-files/

This task is about adding binlog header to GRA_xxx.log file.