As Woofgang says, there is no such thing as 'Gift Tax'.
When your mother dies her estate will be subject to Inheritance Tax (IHT) if its total value exceeds £325,000. (That's the current threshold although, of course, it may well rise in the future).
If her death occurs within 3 years of her gifting you half of your father's estate, then the whole of that gift will be counted as part of her estate when calculating its value for IHT.
If she dies after that time, but still within 7 years of making the gift, then the amount of the gift that is included within the valuation of her estate is reduced on a sliding scale.
If she dies more than 7 years after making the gift then no part of that gift will be included within the calculation.
However if the total value of your mother's estate, at the time of her death, (including the gift to you or part of it, if appropriate) doesn't exceed £325,000 [or whatever the IHT threshold is at that time] then no IHT will be payable anyway.