To view the full headers of an email within the Gmail web interface, you need to select "show original" from the drop down 'More" actions menu on the right. The "show details" button next to the sender details only gives a subset of information, not the full headers.
I tested sending a message from the gmail interface and found there was no X-Originating-IP or any header which gives any sender IP details. This is no surprise. Note that the X-* headers are optional headers and not required by the SMTP protocol.
Even if there was sender IP information in the message, it is largely useless. This is especially the case if you are trying to use it to track someone down. Essentially, you are wasting your time.
Many users don't have static IP addresses. ISPs assign IP addresses from a pool of available IPs.
Many sites use NAT and other mapping techniques so that all hosts on their internal network have non-routable private IP addresses (i.e. 192.168.1.9), but a common public routable IP address.
Users can access Gmail from any Internet connected system which has a web browser, such as an internet cafe, public wireless networks, etc.
Many web connections pass through proxies. From the Gmail perspective, the IP the client is connecting from is the proxy IP, not the IP of the sender's PC
It is fairly trivial to fake an IP address
Even the suggestions of following a legal process, such as court orders forcing Gmail to provide log information etc is a waste of time. While, after much effort and probably significant cost, you may be able to get this information, it is largely pointless. At the end of the day, anybody can obtain a Gmail account under any name. You are not required to prove who you are. There is pretty much no level of assurance that a Gmail user is the person they claim to be. This is the case with many mail providers, not just Gmail. The whole mail environment is inherently flawed in this way and you cannot relay or guarantee anything regarding sender details or mail headers. As the old cartoon depicted, "On the Internet, nobody knows you're a dog".
If you are receiving abusive emails from a Gmail user, your best course of action is to report them to Gmail. They are usually pretty good at shutting down accounts of anyone who is abusing the service. Of course, there is no guarantee this person actually is a Gmail user — as already mentioned, the whole email ecosystem is flawed and it is trivial to make a message appear to be from someone.
Apart from that, all you can do is ignore it. Filter out the offending sender or just hit delete and move on.