Assuming you're talking about a "singleton" (global instance), then the simplest would be to have that method somewhere (anywhere):
@JsProperty(namespace=JsPackage.GLOBAL, name="gantt")
static native Gantt getGantt();
Gantt here would be your native JsType (beware of the namespace="" and name="" on the JsType if you don't want class cast exceptions at runtime; if the type is unknown, then use a Java interface to model it, or use namespace=JsPackage.GLOBAL,name="Object" to make sure that a JS "instanceof" would work)
You could also declare a native JsType representing the global scope (window) with a field to get that "gantt" object, similarly to how Elemental 2 works:
@JsType(isNative=true, name="window", namespace=JsPackage.GLOBAL)
class GanttGlobal {
public static Gantt gantt;
}