GFLOPS to TFLOPS are calculated by GFLOPS = TFLOPS * 1000
This means there are 1000 GFLOPS in 1 TFLOP.
FLOPs represent the number of floating point operations a device can do every second. The higher the FLOPs the higher the theoretical performance. The performance of the device or graphics card is not just dependent on the amount of FLOPs. Parallelism, memory, connection speed can all vary the final performance of a machine. Most of the time you will see FLOPs with the words single, double and half precision. In essence this is just the number of floating point bits that were used to measure the performance with double being most amount of bits and half the least amount of bits.
If you want to convert TFLOPS to GFLOPS click here.