The LayoutChanged event notifies your application once a column is resized or moved by drag and drop. Also, the LayoutChanged event may be fired if the item's position is changed by drag and drop using the AutoDrag property. 

private void LayoutChanged(object sender)
{
}

Private Sub LayoutChanged(ByVal sender As System.Object) Handles LayoutChanged
End Sub

private void LayoutChanged(object sender, EventArgs e)
{
}

void OnLayoutChanged()
{
}

void __fastcall LayoutChanged(TObject *Sender)
{
}

procedure LayoutChanged(ASender: TObject; );
begin
end;

procedure LayoutChanged(sender: System.Object; e: System.EventArgs);
begin
end;

begin event LayoutChanged()
end event LayoutChanged

Private Sub LayoutChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles LayoutChanged
End Sub

Private Sub LayoutChanged()
End Sub

Private Sub LayoutChanged()
End Sub

LPARAMETERS nop

PROCEDURE OnLayoutChanged(oG2antt)
RETURN

<SCRIPT EVENT="LayoutChanged()" LANGUAGE="JScript">
</SCRIPT>

<SCRIPT LANGUAGE="VBScript">
Function LayoutChanged()
End Function
</SCRIPT>

Procedure OnComLayoutChanged 
	Forward Send OnComLayoutChanged 
End_Procedure

METHOD OCX_LayoutChanged() CLASS MainDialog
RETURN NIL

void onEvent_LayoutChanged()
{
}

function LayoutChanged as v ()
end function

function nativeObject_LayoutChanged()
return

Since, the LayotChanged event may be fired on different scenarios, you can distingue the action that previously occurs by storing the ItemFromPoint and/or ColumnFromPoint during the MouseDown event like in the following VB sample:

Dim iItemFromPointMouseDown As Long
Dim iColumnFromPointMouseDown As Long

Private Sub Form_Load()
    iItemFromPointMouseDown = 0
    iColumnFromPointMouseDown = -1
End Sub

Private Sub G2antt1_LayoutChanged()
    If (iItemFromPointMouseDown <> 0) Then
            Debug.Print "Items section changed"
        Else
        If (iColumnFromPointMouseDown <> -1) Then
            Debug.Print "Columns section changed"
        Else
            Debug.Print "Others"
        End If
    End If
    iItemFromPointMouseDown = 0
    iColumnFromPointMouseDown = -1
End Sub

Private Sub G2antt1_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
    Dim c As Long, hit As HitTestInfoEnum
    With G2antt1
        iItemFromPointMouseDown = .ItemFromPoint(-1, -1, c, hit)
        iColumnFromPointMouseDown = .ColumnFromPoint(-1, -1)
    End With
End Sub

The sample displays:

• "Columns section changed" if any change occurs in the Columns section, like moving a column to a new position or resizing the column.

• "Items section changed", if the user drags an item to a new position using the AutoDrag property on exAutoDragPosition, exAutoDragPositionKeepIndent and exAutoDragPositionAny

You can use the LayoutChanged event to save the columns position and size for future use. Use the Width property to retrieve the column's width. Use the Position property to retrieve the column's position. The Visible property specifies whether a column is shown or hidden. Use the ColumnAutoResize property to specify whether the visible columns fit the control's client area.

• Avoiding resizing the control under a specified width, like in the sample:

Private Sub Form_Resize()
On Error Resume Next
    If ScaleWidth / Screen.TwipsPerPixelX > 64 Then
        With G2antt1
            .Left = 0
            .Top = 0
            .Width = ScaleWidth
            .Height = ScaleHeight
        End With
    End If
End Sub

• Using the LayoutChanged event to store the columns proportions manually. The following sample holds the columns proportions when LayoutChanged event is fired. The sample ensures that the proportions are saved only when the user resizes on of the control's columns, not when the user resizes the entire control. The proportions are kept by the Data property of the Column object. The sample can be changed smoothly by using a simple collection to hold the columns proportions instead using the Data property of the Column object

Option Explicit
Dim nFit As Long
Private Declare Function PeekMessage Lib "user32" Alias "PeekMessageA" (lpMsg As MSG, ByVal hwnd As Long, ByVal wMsgFilterMin As Long, ByVal wMsgFilterMax As Long, ByVal wRemoveMsg As Long) As Long
Private Declare Function TranslateMessage Lib "user32" (lpMsg As MSG) As Long
Private Declare Function DispatchMessage Lib "user32" Alias "DispatchMessageA" (lpMsg As MSG) As Long
Private Const PM_REMOVE = &H1
Private Type POINTAPI
        x As Long
        y As Long
End Type
Private Type MSG
    hwnd As Long
    message As Long
    wParam As Long
    lParam As Long
    time As Long
    pt As POINTAPI
End Type

Private Sub Form_Load()
    nFit = 0
    
    onG2anttResize G2antt1
End Sub

Private Sub Form_Resize()
On Error Resume Next
    nFit = nFit + 1
    With G2antt1
        .Left = 0
        .Top = 0
        .Width = ScaleWidth
        .Height = ScaleHeight
    End With
    fit G2antt1
   
    nFit = nFit - 1
End Sub

Private Sub G2antt1_LayoutChanged()
    If (nFit = 0) Then
        onG2anttResize G2antt1
    End If
End Sub

Private Sub fit(ByVal g As EXG2ANTTLibCtl.G2antt)
    nFit = nFit + 1
    With g
        If (.ColumnAutoResize) Then
            .BeginUpdate
            .ColumnAutoResize = False
            Dim c As EXG2ANTTLibCtl.Column
            For Each c In .Columns
                c.Width = c.Data
            Next
            .ColumnAutoResize = True
            .EndUpdate
        End If
    End With
    waitToProcessMessages
    nFit = nFit - 1
End Sub

Private Sub onG2anttResize(ByVal g As EXG2ANTTLibCtl.G2antt)
    Dim c As Object
    With g
        If (.ColumnAutoResize) Then
            For Each c In .Columns
                c.Data = c.Width
            Next
        End If
    End With
End Sub

Private Sub waitToProcessMessages()
    Dim m As MSG
    While PeekMessage(m, 0, 0, 0, PM_REMOVE)
        TranslateMessage m
        DispatchMessage m
    Wend
End Sub