How can I characterize gm vs W/L in ltspice. Can anyone provide the testbench explaining the proper steps? 

This is really more of a design question than an LTspice question. You might have better chances of a useful answer by going to a forum focused on IC design.

 

But, I want the characterization to be done in ltspice

You can .STEP both W and L, or a separate parameter equal to their ratio W/L.

 

Do you know how to simulate transconductance, gm?

 

Andy

I want the gm to be at y-axis & W/L in x-axis... Is this possible in ltspice? 

Yes.

Tell me how? 

That depends on how you do the simulation.

 

.STEP command?

I do mean that seriously.  How to get W/L on the X-axis depends on how you run the simulation.  If you used a .STEP command, it might plot that way automatically.  Or, you might need to right-click on the X-axis and change the Quantity Plotted for that axis.  Changing the Y-axis to be the transconductance is probably easier but it is probably a formula involving current and voltage.  But it all depends on how you set up and run your simulation.

 

Andy

 

I understand that, but you must first convince yourself that you have a method which can gather the data you seek. THEN, it will be time to see if LTspice is up to the task. You're putting the cart befor the horse. That never works out very well.