mywidget->setGeometry(0,0, parentWidget.widht(), 100);

You shouldn't be wanting to do that. Just put the appropriate layout and setup your widgets in the designer. If any of them is fixed size put that in the appropriate properties of the object (the right side of the designer, after clicking on an object).

Kind regards.

But If I want use not fixed size widgets? How can I resize Ui::WindowTitle titleUi; custom widget?

I want to set size dynamically, for example:

mywidget->setGeometry(0,0,  parentWidget.widht(), 100);

What about:

SandBox::SandBox(QWidget * parent)
    :  QMainWindow(parent, Qt::FramelessWindowHint)
{
    ui.setupUi(this);

    Ui::WindowTitle titleUi;  //< This could be put as a class member as well
    titleUi.setupUi(centralWidget());
}

You don't seem to be doing anything special with your WindowTitle class, so no need to derive QWidget only to set up a form.

Kind regards.

Now WindowTitle instead of Frame and Sandbox intead of GWindow

class WindowTitle : public QWidget
{
    Q_OBJECT

public:
    WindowTitle(QWidget *parent = 0);
    ~WindowTitle();

private:
    Ui::WindowTitle ui;
};


WindowTitle::WindowTitle(QWidget *parent)
    : QWidget(parent)
{
  ui.setupUi(this);
}

WindowTitle::~WindowTitle()
{

}

#include <QtWidgets/QMainWindow>
#include "WindowTitle.h"
#include <QGridLayout>
#include "ui_SandBox.h"

class SandBox : public QMainWindow
{
    Q_OBJECT

public:
    SandBox(QWidget *parent = 0);
    ~SandBox();

private:
    Ui::SandBoxClass ui;
    
};

#include "SandBox.h"

SandBox::SandBox(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);
  
    setWindowFlags(Qt::FramelessWindowHint);

    QWidget * widget = new QWidget;
    QGridLayout * layout = new QGridLayout;
    WindowTitle * w = new WindowTitle;
    
    w->show();
    layout->addWidget(w,0,0,1,1);
    widget->setLayout(layout);
    setCentralWidget(widget);
}

is FrameObject?
Where and how is it defined?

into GWindow header

class GWindow : public QMainWindow
{
    Q_OBJECT

public:
    GWindow (QWidget *parent = 0);
    ~GWindow ();

private:
    Ui::SandBoxClass ui;
    Frame FrameObject;    
};