Gwilym started volunteering as a schoolboy for Museum of London Archaeology Service (MoLAS) in the latter half of the 80s. In 1989 he moved to France to work for l’Unité d’Archéologie de la Ville de Saint-Denis where he continued working full- and part-time, while studying Archaeology at the University of York, over the following five years. He worked and supervised on medieval urban sites, including a cemetery, churches and defences.

In 1994 he started work for OAU where he worked until 1997, carrying out watching briefs, desk-based assessments and project set-up on the Mayenne Project. He supervised work at Royal Hospital Chelsea, Kew Gardens and on medieval sites in Oxford and London. During his time at OAU he completed the Post-Graduate Diploma in Field Archaeology.

Following a Masters at UCL he returned to field archaeology working for MoLAS, where he worked at Spitalfields, and Cambridge Archaeological Unit, where he supervised a number of medieval sites including an excavation at Ely Cathedral. He has also worked abroad in Bulgaria (University of Nottingham), Bosnia (International Criminal Tribunal for the former Yugoslavia) and Kuwait (UCL).

In 2001 he moved to Sweden to work for Malmö Kulturmiljö and subsequently Riksanikvarietämbetet on prehistoric and particularly Vendel period through to medieval sites. He has worked at JMHS since 2006, where he has carried out interventions on post-medieval gardens, rural medieval monastic and industrial sites, Romano-British farmsteads and prehistoric settlements and barrows.