According to this blog, Google hired Sean Egan, main developer of gaim, from the gaim website, seems like Sean is already working with Google for quite some time. Google have its own instanst messenger just like Yahoo and Microsoft, google's IM is very new (and still in bete version), compare to Y!Messenger and MSN, which already available on the IM market for many years. Google's messenger is base on a open protocol--Jabber.
Since Google Talk client only available on Windows platform now, other operating system users have to use 3rd parties IM client. I am using AdiumX on OS X to connect to Google Talk, it is working fine. AdiumX is base on gaim engine. (The setting of Jabber for AdiumX and Gaim is a bit different, please refer to the Google talk help center for gaim and adium)
The advantages of hire opensource developers and using opensource software, allow google to go into some areas like instant messager with less effort. Google just provide the IM client for windows but software on other platform (eg. OS X and Linux) is already available. Take a look at Yahoo! Messenger and MSN, their clients on non-windows platform is still struggling, is either the version are too old (compare with the software available on Windows platform) or is not working properly(Yahoo messenger have some problem executing on OS X 10.3 Panther, after patched with os online update).
Google hired Firefox lead developer-- Ben Goodger early this year.