TPS54260 gain margin value

Hola TI,

I have a gain margin value issue for TPS54260 simulation result：

Below is my application,

Vin = 21V

Vout = 12.5V

Iout = 0.2A and 0.1A

I used Webench to simulate the above condition but the final print report didn't show the gain margin value only phase margin！

Can someone tell me why this happened？How can know the gain margin value？

Thanks.