My first post!
I was in a situation where I had to do this. I lent a friend my iPod Touch to try for a few weeks... I then realized my GMAIL account was on it!
So...
1.) Web interface: Changed the GMAIL pword.
2.) On the Droid: Launched the GMAIL app -wouldn't sync. Good.
3.) Searched thru the GMAIL app to reset to my new pword. No luck.
4.) Noticed a new notification, swiped it down, viola! Enter new pword!
In short, don't look in the app itself to change the pword, rather expect a notification to re-enter the pword!
Hope that helps...