Hello. I don't know the GRT for my 33T off hand but there is a simple formula:
L = Length (not including bowsprit) = 32.75
W = Beam = 11.75
D = Draft plus water line to deck & hull joint = ~7.5
.57 = Factor used for all sailboats when calculating Gross Registered Tonnage
GT = ( L x W x D x .57 )/100 = ~16.5 GRT
That should get you pretty close. For NET Tonnage or more detail see this article:
https://www.dco.uscg.mil/Portals/9/DCO% ... 123757-680