The easiest way to handle dependencies is to let the Application Manager do it for you. For example, you can try to install the GPE-Calendar .deb file manually, but as you noticed, it depends on a lot of other .deb files, so this quickly becomes a hassle.

Instead, you can add the kernel-concepts repository to the Application Manager:

Web address: http://downloads.kernelconcepts.de/maemo3
Distribution: bora
Components: free

Then, select gpe-calendar from the list of applications. The Application Manager will automatically install any needed dependencies.

Unfortunately, there is a large (and growing) number of repositories, so finding the right repository for a particular application is a hassle of its own. There is a wiki page with a list of repositories that may make this hassle a little easier:

http://maemo.org/maemowiki/ApplicationRepositories