It appears that the GT4 has an odd offset. I'm thinking that it will be easier to go with new wheels. I'd hate to buy a set and find they don't fit.
I'm not sure if the standard celica and the GT4 share the same stud pattern and offset but the only other car with a similar offset is a VW Golf. I think the Hyundai is the same also, so perhaps I should find a Hyundai forum and see if someone is selling their fullysik 17's so they can put 19's on their Excel