1. Till probate is taken on a Will, it has no value more than a scrap paper,
2. When a property has already been gifted through a registerd gift deed to your father, your grandfather can not write a will for that property,
3. Similarly, the property which belongs to some onbe else can not be willed by your grandfather,
4. The above are enough grounds for you to challenge the so called will alleging it to be with fake signature of your grandfather or signed with coercion,
5. Let your father file a partition suit claiming equal share on all the properties left behind by your decsaed grandfather.