A gamete is an indispensable "part" of a human being. But for the gamete, there would be no human being. Ten fingers are not an indispensable "part" of a human being. Your reasoning seems to me rather facile.
How so? What about human beings who cannot produce gametes? Are they not human because they lack that property? But you say, "but for the gamete, there would be no human being.". That is true but irrelevant because you are conflating the hypothetical potential of a gamete to become something different, i.e. an actual human being, a different category of being (by being united with another gamete of the opposite sex) with an actual human being with potential. When an ovum is fertilized both it and the sperm cease to exist as such and become a single new entity, an entirely new thing; a new human being, one who has never existed before and will never exist again. Though a gamete is a necessary part of the creation of a human being it does not follow that it is the same category of thing as a human being.
It is a contradiction in terms to say that the potential and the actual can exist simultaneously in any thing. It is impossible. An actual gamete with potential and an actual human being with potential are two different kinds of being entirely.
Cordially,