Tg = Gift tax on the estate. (And the estate transfer occurs only after the gift tax is paid).
In any case that needs to be paid. So (1-Tg) will always apply.
Now if the DONOR decides he likes you the DONEE too much, and decides to pay the gift tax on your behalf - then the numerator changes to (1-Tg + Tg*Te) - because you get an additional amount of money (as DONEE, recipient) on which you end up paying ONLY the estate tax.