CPF, Chicago-1990. GCF, Atlanta-2002.
The CPF is a difficult test of preservation knowledge. Book study is essential -- get the current Study Guide from PPFA, buy or borrow the recommended books, and have at it. Veteran framers often fail their first try, because they think they don't need to study. IMHO, that's why the failure rate is about 40%.
The GCF is more comprehensive & skill-based, but less difficult than the CPF. Candidates are required to bring three completed frames to the exam, complete three timed, observed tasks, and complete a brief written exam. The hands-on portions are by nature somewhat subjective, which is why PPFA didn't include them in the original CPF exam. But FATG seems to do a consistently good job of grading.