Using GTID to attach an asynchronous Slave sounds promising. Lets have a look at the two existing GTID implementations and their integration with Galera.
GTID@MariaDB
There is one GTID used by the cluster and every node increments the common seqno by itself. This works well as long all transactions are replicated by Galera (simplified InnoDB). Because Galera takes care of the Commit Order of the transactions on all nodes. So having identical GTID/seqno from the start there are no problems.
node1> show global variables like 'gtid_binlog_pos';
+-----------------+---------+
| Variable_name | Value |
+-----------------+---------+
| gtid_binlog_pos | 0-1-504 |
+-----------------+---------+
node2> show global variables like 'gtid_binlog_pos';
+-----------------+---------+
| Variable_name | Value |
+-----------------+---------+
| gtid_binlog_pos | 0-1-504 | …
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