Is there an easy way to display Atlassian Gadgets within a Drupal node?
Well, taking a look at the Jira/Atlassian web site, they are based in Java technology. Without going into too many details, because I'd have to learn how their proprietary environment works to advise you further, it is possible to embed Java applets in a Drupal page, in pretty much the same manner that one embeds a YouTube video on a Drupal page: just embed the markup in the same manner as you would any other web page, just make sure that the Drupal page's Input Filters are set to not strip out your embedding tags (such as using the 'Full HTML' Input Filter.)

However, this just gets one of their Java Applets on the page. It's not interacting with any information in your Drupal installation. In order to get your Java communicating with your Drupal installation's data, you need to install something like the Services Module into your Drupal installation hosting these Jira/Atlassian Java Applets, plus the Services REST server, and then create an API within the REST Services Framework that either mimics what the Java Applet expects for it's communications, or custom modify the Java Applets to communicate thru the API you created within Services.

So, no, there is no easy method if you want these Java Applets communicating with your Drupal installation's data. If you just want to display them, AND they support simple embedded like most Java Applets, just use the appropriate Input Filters when placing the Java Applet's embed script in the Drupal page.