wxPanel widgets always crowd together on the up-left side of the frame

Hi People,
I'm a newbie to wxPython, and now is having a problem on the layout of widgets on a Panel within a Frame.
No matter which Sizer I use, and what kind of layout format are adopted, all the widgets always crowd together on the up-left side of the frame, and left much spaces unused on the rest part of the frame .
Below is all my code, please help.

Thanks In Advance,
JW


import wx

class AuditFrame(wx.Frame):
def __init__(self):
wx.Frame.__init__(self, None, -1, "TJBY Auditor",pos=(150, 150), size=(500, 200))

self.Panel = wx.Panel(self,-1,(0,0),(500,200),style=wx.SUNKEN_BORDER)
self.PanelSizer = wx.GridBagSizer()

self.btnSelect = wx.Button(self.Panel,-1,"Select Invoices")
self.PanelSizer.Add(self.btnSelect,(0,0))

self.txtOpenPath = wx.StaticText(self.Panel,-1)
self.PanelSizer.Add(self.txtOpenPath,(0,1))


self.btnCheck = wx.Button(self.Panel,-1,"Start Checking",(0,0))
self.btnCheck.Disable()
self.PanelSizer.Add(self.btnCheck,(1,1))

self.TxtCtl = wx.TextCtrl(self.Panel,-1)
self.PanelSizer.Add(self.TxtCtl,(2,0),(2,2)) #How to enlarge the TextCtrl?
self.Panel.SetSizer(self.PanelSizer)


app = wx.App(False)
MainFrame = AuditFrame()
MainFrame.Show()
app.MainLoop()