Hi and welcome to the board.
Assuming the changes to column G will be manually entered (or cut/copied & pasted - NOT a formula updating) then something like this in the worksheet module should get you started.
Right click the sheet tab & choose View Code. Paste this in the sheet module.
Private Sub Worksheet_Change(ByVal Target As Range)
'Specify to work only with single cell changes
If Target.Count > 1 Then Exit Sub
'Specify to work only when a cell in column G changes.
If Target.Column <> 7 Then Exit Sub
If Target.Value > 0 Then MsgBox "HEY!!! Column G has a value greater than zero."
press Alt+Q (to close the vb editor) and make an entry in your sheet to check it out.
If your column G changes happen because of formulas updating, (instead of manual entries) let us know and we can fix it to use the sheet calculation event instead of the sheet change event.
Have you looked at the worksheet and selected the Tab and used the right mouse button and then View Code. You can select some options from there, Change the first drop down to Worksheet and the second drop down to Calculate or Change and place in an if statement.
Or would a normal if statement be useful on the sheet to determine if the value is greater than Zero.
What do you want to Happen to the Cell?
PrivateSub Worksheet_Change(ByVal Target As Range) If Target.Count > 1 ThenExitSub If Target.Column <> 7 ThenExitSub If Target.Value > 0 Then MsgBox "The value added is greater than Zero." EndSub