Are there any Ubuntu based applications that will allow Google Talk users talk to Skype users?
In short the answer is no, not because there aren't many great applications available for Ubuntu, but because Skype is a proprietary software owned by Microsoft, and they do not provide an API to allow for third party clients to use Skype in the manner.
I had the same problem because I use several operating systems at home and at work and the solution I've found is very simple.
First of all there is no special app for Ubuntu, because you can do what exactly what you need by merging your accounts and contacts in skype.
Here's a simple step by step guide.
1 - Connect your gmail account to msn (Create a Windows Live ID using an Existing Email)
Now that you have all of your gmail contacts on your Live account, all you have to do is:
2 - Download and Excecute the latest version of Skype app
3- Merge your contacts
you can chat with your google contacts, using only one client (skype).
Hope this will help you. Best regards.