Currently if MySQL 5.6 is running with GTID enabled, replication to MariaDB does not work, it breaks on the first GTID-related event (e.g. 'Previous_gtids' or 'Gtid') with
If it's at all possible, I think it's fairly important to be able to replicate from MySQL 5.6 with GTID to MariaDB, as one of reasonable migration scenarios is that a production has the main pair of MySQL 5.6 master and slave (using GTID), and a second MariaDB slave is running in parallel.
Of course it would be old-style replication, not using MariaDB GTID.
That said, replication from MySQL 5.6 GTID to MySQL 5.6 without GTID does not work either (it fails with "The slave IO thread stops because the master has @@GLOBAL.GTID_MODE ON and this server has @@GLOBAL.GTID_MODE OFF"), so maybe it's impossible.