BlackSunshine,
Is Unified Messaging setup to go somewhere else already? If not then the answer is yes it will work with G-Mail and other hosted solutions, but maybe not with Enhanced Integration; Forward Only and Copy and Forward always seem to work.
If the answer is yes then you will need to create a Relay Account to the server it is currently talking to that will forward the messages on to their G-Mail account.
There are many posts on how to do this already on the forums. If you have an issue figuring it out then repost it here.
Thanks,
TE