One possibility is to use a GoTextNode that has its AutoResizes property set to false, so that you can set the GoTextNode.Background.Size to whatever you want. You may also want to set the GoTextNode.TopLeftMargin and .BottomRightMargin properties, to adjust the size of the space around the Label inside the Background.
tn.Text = "some text";
tn.TopLeftMargin = new SizeF(20, 35);
tn.BottomRightMargin = new SizeF(20, 5);
GoTriangle tri = new GoTriangle();
tri.Resizable = false;
tri.A = new PointF(0.5f, 0);
tri.B = new PointF(0, 1);
tri.C = new PointF(1, 1);
tn.Background = tri;
tn.AutoResizes = false;
great thx you!