What you're saying might be true, but some of us, like me, like to understand what's under the hood. Learning javascript from scratch, in my opinion, is a great way to fully take advantage of what javascript has to offer.
At that point you stop, thank the deity of your choice for the existence of people who abstract away browser incompatibility so you don't have to worry about it, and move on to something else.
GWT may not be the solution for you if you aren't a Java guru, but if you are, it is a godsend if you need to develop ajax ria apps quickly and you don't want to learn (deal with) all the browser incompatibilities and js nuances.
Learning the former is good for you. jQuery will help you do that.
Learning the latter is actively bad for you. You're learning a technology that is already obsolete.