Google Gears is open source software, licensed under the New BSD license. Generally speaking, this license is very permissive. You should, of course, always consult an attorney if you have any questions about software licensing.
There are generally two ways to use Google Gears: by embedding the API or runtime software in an application you distribute to end users, or by writing a web application which makes use of installations of Gears on end-users' computers.
The New BSD license allows you to embed the Google Gears runtime or API in your software, with very minimal restrictions. There are no restrictions on web applications which make use of Google Gears installations on users' computers.