Here's a simple method that you can use:
Say you are just using a Page type. The code that does the work will be in /mysite/Page.php.
If you add a method to the PageController in that file that returns the HTML you require, you will be able to access that code in all of your Pages that are of this type.
Here is a simple method to get you started.
public function getGCalHTML()
$html = '<iframe src="http://www.google.com/calendar/embed?src=kse%40neteffects.no&title=Kalender%20for%20Kim%20Stian&height=614" style=" border-width:0 " width="640" frameborder="0" height="614"></iframe>'
Then, in your Page.ss file, you simply call this method to output the html that is returned:
Note, Div's are not required. The $getGCalHTML will output whatever it is that make that method return.
The above example is a good way for you to understand how it all works, but probably not ideal for production, as it will show your calendar on every page that is set as a normal page in your admin. You probably only want this on one page, in which case you need to create a new page type just for this (e.g. CalendarPage). The [url=http://doc.silverstripe.com/doku.php?id=tutorial:1-building-a-basic-site"]tutorial on making a custom home page[/url] is exactly what you need to look at here.
Hope that helps.