If the donee dies before acceptance, the gift is void. ... A transaction of gift in order to be complete must be accepted by the donee during the lifetime of the donor.

in such a case donor can dispose of property as he desires

there is no gift if not accepted by the donee

There is no such law that can be presumed for under the facts and circumstances of the your case. Gift is a transfer inter vivos i.e., both the donor and the donee must be present and alive to complete the transaction. if one of the person dies before execution, there can be no valid transfer. since, the donee dies before the execution of the gift deed, the same cannot be treated as a will.

If the donee dies before acceptance then gift is void. There is no question of conversion of a deed which has not yet come in existence.