Step: 1

Open gmail and open the email you want to trace the sender and click on the 3 dots and click on the "Show Original" as shown in the screen shot below.

Step: 2

On the new tab of "Original Message" you will see the SPF: Pass with the IP address of the sender as shown in the following screen shot with red marked area, get the IP and use IP to Location tool to find the sender, thats it.