For a current account of GAL consult the Research Handbook on Global Administrative Law, edited by Sabino Cassese, who has also edited (with others) the GAL Casebook (now in its 3rd edition).
A GAL bibliograhy has been published in Law & Contemporary Problems (Vol. 68, No. 3-4, March 2005). An updated version by Maurizia De Bellis was published in June 2006.