How can I know the IP of a GMail user upon receiving an email from his end? I know it’s possible to see it with other mail hosting sites but not GMail. Would you please help?
All true, although you should always bear in mind that headers can be spoofed (faked), so use the information with care!
If you download your emails through Thunderbird, Control+u will do the trick as well, or you can go view --> headers --> all
I was actually going to suggest the article which Aibek linked to. In that article, Stefan pointed out the additional information stored within the message's header including the sender's IP, email provider and any desktop software used.
We actually did an article on this earlier. Please refer to the link below. The article looks at how to trace emails both for Yahoo Mail and Gmail.
How To Trace Your Emails Back To The Source