In general, we don't recommend using iframes. That said, there are two possible answers to this question.
If you want to embed a dashboard into your website, i.e. having different pages with different dashboards, the answer is yes.
It's possible to embed a (Geckoboard) dashboard into an iframe using a Sharing Link, the code for your iframe should look similar to this (although you will probably want to add height and width parameters):
If you want to embed online content into your dashboard via an iframe, the answer is no.
It isn't possible to embed content in Geckoboard this way. Here you'll find the different methods to get data that we support on to Geckoboard. You might find particularly useful the Image widget, if you want to embed an image and the Text Custom widget if you want to embed messages and/or other HTML content.