Here an update to this answer:

Hi Atif, 

Maybe you want to have a look to the following chapter in "A Practical Guide to Trade Policy Analysis" (chapter 3):

Chapter 3: Analyzing bilateral trade using the gravity equation

http://vi.unctad.org/tpa/web/vol1/ch3.html

Also, you might want to have a look to the Shepherd review of gravity models, available here:

http://www.unescap.org/sites/default/files/GravityUserGuide-REVISED-02.pdf

Best regards,

Laura

Also, it might be useful for you to see data organisation and follow the dofiles that I made available at my paper entitled "Port facilities, regional spillovers and exports: Empirical evidence from Spain"

http://onlinelibrary.wiley.com/doi/10.1111/pirs.12127/abstract

Data and supplementary information are available at:

http://onlinelibrary.wiley.com/doi/10.1111/pirs.12127/suppinfo

I hope that this helps,

Laura

 Thank you very much Laura. It will help me a lot.