The group panel has different style options. The default (modern) style shows a round rectangle with the group label. The flat style shows a solid line with a label. To use the flat style, simply use:

Style.addStyleNames(group, "f_panel_flat");

The modern vs. flat style:

It's also possible to hide the border of modern styled group, with:

Style.addStyleNames(group, "f_panel_noborder");