You need some JavaScript and CSS to do this.
The GUI constructor can be passed a paramaters object. You can tell the control not to be autoplaced. You can also attach an element ID to make styling easier
var gui = new dat.GUI( { autoPlace: false } );
gui.domElement.id = 'gui';
And then the CSS to place it can be something like this:
#gui { position: absolute; top: 2px; left: 2px }
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…