I'm not a user of google gears up till now, so what I said might be incorrect.
Google gears will cache information or data on your hard disk, and thus the data could be accessed offline. Not only web pages are cached, but many application data could be also cached. The behavior is like a proxy server but local only and powerful.
Online or offline might be indistinguishable, or not important. One might continue to work without network for certain amount of time. An ordinary web visitor might not sending requests every second. Instead, it might be several minutes before he/she jumped to the next page. There are types of services that requires continuous interactions, while there are more services, like archives or documents, that could be viewed offline.
It might be possible to make use of others' cache. For example when the server is down, you are referring to a product manual on that website. This is like a P2P network proxy. There are problems like privacy, consistency, and reliability though. Currently, the function of google cache is quite limit.