The papers were entitled "Medieval Gruit Ales Revisited: New Theories about Old Beverages"; "Microbial Susceptibility of Hopped and Non-Hopped Ales"; "Hildegard's Cerevisiarius: Replicating the Ales of Eibingen Abbey"; and "The Gallic Origins and Spread of Beer-Hopping."
I attended the Congress but did not make it to that panel (there were nearly 50 panels going on at the same time and I chose to go to a different one--in retrospect the medieval ales one probably would have been more interesting). I have no idea what "gruit" means.
Gruit (alternately grut or gruyt) is a herb mixture used for bittering and flavouring beer, popular before the extensive use of hops. Gruit or grut ale may also refer to the beverage produced using gruit.
Historically, gruit is the term used in an area today covered by the Netherlands, Belgium and westernmost Germany. Today however, gruit is a colloquial term for any beer seasoned with gruit-like herbs.
Gruit was and is a combination of herbs, commonly including sweet gale (Myrica gale), mugwort (Artemisia vulgaris), yarrow (Achillea millefolium), ground ivy (Glechoma hederacea), horehound (Marrubium vulgare), and Calluna heather (Calluna vulgaris). Gruit varied somewhat, each gruit producer including different herbs to produce unique flavors and effects. Other adjunct herbs include juniper berries, ginger, caraway seed, aniseed, nutmeg, cinnamon, mint and occasionally hops in variable proportions (although gruit today is often sought out for lacking hops).
https://en.wikipedia.org/wiki/Gruit