Is this what you meant?
Needs to go into the sheet's (code)
Code:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim wb As Workbook, ws As Worksheet
Set wb = ThisWorkbook
Set ws = wb.Worksheets("sheet1")
If Not Intersect(Target, Range("G7")) Is Nothing Then
ws.Range("H7").Value = ws.Range("H7").Value + ws.Range("G7").Value
End If
End Sub