How do I force Safari 5.1.7 to ask to remember my Gmail password?
Copy the code from this page http://ostermiller.org/bookmarklets/rememberpass.js into the address field of a new bookmark, effectively creating what's known as a bookmarklet.  

Once created, click on the bookmarklet while on the gmail login page.  Wait for the page to refresh and then enter your password.  If done correctly, the browser will ask to remember your password.