Andrew is correct, the last Ferrari model whose serial number ended with the GT suffix was the 330 GT 2+2 (the last one was 10193, built mid-1967). But Ferrari used many serial number suffixes, not just GT.
Every road car serial number, starting with 001 S, had an S at the end of it; the last one being 0151 S. Some of those had an ES suffix, some had EL, and some had just an E or a T. It's likely that the S stood for stradale (street) since the competition cars of that era had "C" as a serial number suffix (standing for competizione).
All of the 250 Europa serial numbers have EU for the serial number suffix; but the EU suffix began before the 250 Europa was introduced. All Europa GTs have GT as a serial number suffix.
All of the old Superamericas had SA serial number suffixes; the 500 Superfasts had SF as the suffix. The first Ferrari serial number to go without a suffix was 3765, a 330 LM-style 400 Superamerica.
These days, serial numbers don't get suffixes because the serial numbers are part of the VIN. But the serial numbers themselves are still identified on various parts of the cars (the matricola plate on the dash, the chassis plate in some engine bays) - wouldn't it be cool if those serial numbers got suffixes?