GWT translates Java into JavaScript. Nothing more. When you think about layouts, think HTML and CSS first. You can create a very complex layout using Ui:Binder without a single line of Java code. You will need GWT for handlers and such.
GWT does offer some widgets - like a LayoutPanel - for convenience. So you can use it as a container for your form, possibly split into a couple of layers (left and right column), with each layer containing an HTMLPanel (or ScrollPanel and then an HTMLPanel inside of a ScrollPanel). Then you add all your fields into each of these HTMLPanels.