The GRAT tool chain -- efficient (UN)SAT certificate checking with formal correctness guarantees. We present the GRAT tool chain, which provides an efficient and formally verified SAT and UNSAT certificate checker. It utilizes a two phase approach: The highly optimized gratgen tool converts a DRAT certificate to a GRAT certificate, which is then checked by the formally verified gratchk tool. On a realistic benchmark suite drawn from the 2016 SAT competition, our approach is faster than the unverified standard tool drat-trim, and significantly faster than the formally verified LRAT tool. An optional multithreaded mode allows for even faster checking of a single certificate.