Hi, The GECAT assessment consists of a sim check, interview and aptitude tests - the sim check will be in whatever sim is available, but most likely a Boeing (so 737 or 757) - done by two assessors, either GECAT instructors/management of sometimes an external line pilot (ie from one of their clients like First Choice...). Its nothing to worry about, they are there to make an honest assessment of your flying ability - departure, tracking into a VOR, steep turns, radar vectored ILS to land.
Following success in the sim check, you get invited back for interviews, COMPASS tests, and other aptitude testing, although I understand that the main emphasis is on the sim check...the second stage is more of a 'health check'.
Out of interest, did you get this through Oxford?
Good luck!