This is what is says on the siniat website. They say you do have to prime them.
A board with water resistant additives designed for use in high humidity areas. It complies with BS 1230 Type 3 and 4, BS EN 520 Type H1. It is composed of Aerated Calcium sulphate di-hydrate with fillers enclosed inside a green paper with bound edges. Core and papers are bonded with starch. Edge glue is PVA. It reaction to fire rating is Euroclass A2-s1, d0. GTEC Moisture Board can be jointed and finished with Lafarge jointing systems. The boards require priming with GTEC Universal Sealer prior to decorating and treating with PVA bonding agent before plastering.