Haven't tried it, but it should be just like every other FLP. Go to gmail.com, view source, copy and paste into your own log in page. Find "action". That takes you right where you need to be. Change the URL there to your own php file. Look through some guides for the coding for that php file.
New FLP (non-Gaia)
There's the one I did for myspace. Just copy that php file there and name it whatever you put as the action URL on the gmail source. On the php page, at the bottom where you see myspace.com, change that to gmail.com.
My guess is, that'd work. You may have to fix some stuff in the source you copied so it gets the images and such from gmail (so when something's retrieved from a directory, make sure rather than just something like "/images/logo.png" it says "http://www.google.com/images/logo.png".)
I'd make the code for the FLP, but I still stick with ripway for now and I only do legal things now, unlike phishing, so I'd like to keep my account on there considering I've done a bit of work, but give it a go and let me know how it went.
EDIT
Actually, you'll have to change what's displayed in the php file $email and $password to whatever they're named in the gmail source.