In this case you could use conditional formatting:
Go to cell G5
From the menu choose Data => Validation
Validation criteria:Custom
"Ignore blank" checkbox: Unchecked (!)
Formula: =($E5+$F5)>0

Choose the "Error alert" tab and into Title field put something like "Input
restricted", into Error message field something like "The total of column E
and F must be > 0 before you can input data here!"

Push OK

Now G5 is "locked" (no input possible) unless the sum of E5+F5 is greater
than 0.
Copy G5 up to N5 and down as may rows as needed.

Cheers

Joerg Mochikun