There are several tutorials on how to use Thunderbird on Gmail, and I think there's even one in the forum somewhere.
That's not what you asked for but I thought I should mention it
One way to handle mail when distrohopping and actually between Windows and Linux is to put the Thunderbird profile on a separate partition (separate home or whatever) and then share it between all the OSes you use
I remember describing it somewhere but I write so much so I don't find it
What you do is simple - you move the Thunderbird profile folder to a separate partition as mentioned and the you use the Thunderbird profile manager to reconnect to the profile - google Thunderbird profile manager and you'll immediately find out how
You have one way to convert Tbird to Gmail herehttp://kb.mozillazine.org/Importing_and ... _your_mail
and that page also contains a link to how you move your profile