You should decide whether you want the device to be tethered to a computer, or if it's important that it be able to function on its own.
You basically have two options:
1) Write some sort of computer application (in whatever language you want - Python, Processing, C, Java, Perl, etc.) that grabs a webpage (Gmail in this case), scans it for certain data (your heade info) and sends data out via a USB/serial port to the Arduino. The Arduino then does whatever you need with it.
Pros: Analyzing a web page is likely easier with a "real" PC-based application than with the much more limited Arduino. For that matter, there may be some sort of Google-provided API that may make talking to Gmail even easier.
Cons: You need a PC (with this app installed) to always be connected to the Arduino for your project to work.
2) Connect the Arduino to the internet (via an EtherShield, or WiFi shield, etc.), and then have the Arduino grab the webpage and process it.
Pros: The project is computer independent, you can plug it into any network (or hook up to any WiFi hotspot, etc.) and it'll work.
Cons: Likely much harder to code, though there may be libraries out there for the Arduino to make it easier.
Personally, if I were in your shoes, I'd start with option 1 (using whatever language I'm already familiar with), get everything else working, than consider switching to option 2. YMMV.