Since I can't edit the wiki page, I'll put the data here. I just bought an Opel GT that I plan to ecomod thoroughly.
Make Model Year CD Frontal Area CDA
Opel GT 1968-1973 0.398 18.5 sq ft 7.16
I don't have the height/width figures, the other information is from
OpelGT.com - The Classic Opel Community, an excellent forum for Opel GT owners