Google designed an iPad-specific site. From the Official Gmail Blog:
When the iPhone and Android devices came out, we started building for advanced mobile browsers and optimizing the Gmail interface for touchscreens, culminating in the Gmail for mobile HTML5 web app. As portable devices continue to evolve, we're excited about the upcoming wave of tablet computers and the possibilities they bring.
With today's release of the iPad, we're launching an experimental two-pane user interface to take advantage of its large touchscreen and tablet form factor. Building upon the Gmail for mobile web app, this new interface displays your conversations on the left and your messages on the right hand side.
All the features of the Gmail web app that you're used to, such as offline access and aggressive caching to reduce latency, are present in the iPad version. Tablet devices are still very new, so expect changes as we continue to optimize for this new format. We'd like to hear any ideas you may have on how we can make Gmail better for the iPad so don't hesitate to let us know via the 'Send feedback' link at the bottom of your inbox on iPad.
To try out Gmail out on the iPad, just go to gmail.com in Safari. For quick access, try creating a homescreen link. Please note that the new interface is only available in US English for now. You can always access Gmail through the native Mail application using IMAP as well.