|I am running 18x9 and 18x10.5 on my Z.
Front offset is +37 and rear offset is +44
You are correct in assuming you have 18x8.5 and 18x9.5 as the tires do not look stretched on your photos. For a 18x8.5 GT3 wheel the offset will be +35 for the 18x9.5 should be +38 or +40.
Hope that helps.
It's never "DONE", just finished enough to drive :-) - BigTDogg