font-size: 80% !important;
This would be one way that would likely work, but it isn't really addressing the root of the problem. What does the GWT CSS look like?
I am guessing you are having a CSS conflict somewhere. Your CSS rule as indicated above, doesn't have as much weight as a rule within the GWT CSS. Take a look at this site: http://css-tricks.com/specifics-on-css-specificity/
This is a pretty good article that can help to clear up the root cause of some CSS conflicts. The formula that this site talks about really opened my eyes when I first discovered it (the formula, not the article).
Basically, you add up a "score" for each rule in your CSS. The rule with the highest score wins and gets to style the element.
The font-size of the body would be 50%.