How to calculate the gate count of FPGAs?

FPGAs are fundamentally different. You cannot get a reliable "gate count" from an FPGA. The only meaningful figures are LUT count and register count.

Thank you for your reply Tricky .... But in ACEX 1K and APEX 20K i am getting the gate count from datasheet. I am wondering for other FPGA families

ACEX and APEX are very old devices (like 20 years old!) when gate count probably had a little more meaning to an FPGA. Now, because a single LUT can have between 4 and 6 inputs, it can implement anything from 1 gate to many of gates in a single LUT. So its very difficult to get a gate count equivolence.