My grandfather Moosa gave his grandson Siddeeq a piece of land because he was an orphan. My grandfather Moosa died and then his grandson Siddeeq also died leaving only one son Zaid.
My question is what will happen to this piece of land? Will it go to Zaid or it will go back to sons of Moosa (my grandfather) and it will be divided between them equally and Zaid will only receive a portion of the land as he will represent one (dead) brother only.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Since your grandfather offered this piece of land to his grandson while he was a sane with all his senses functioning properly, then it became the property of the grandson and it will be added to his inheritance and will be divided among his heirs. His brothers (brothers of the grandson) will not inherit him because he had a son (who will inherit him).

Allaah knows best.