Short answer. Yes, potentially. Article 750 allows a donation of all present property only if the donor reserves sufficient means to support himself and relatives he is legally bound to support. Without that reservation, the donation shall be reduced on petition of any person affected, which could include your uncle.
What the law says
The donation may comprehend all the present property of the donor, or part thereof, provided he reserves, in full ownership or in usufruct, sufficient means for the support of himself, and of all relatives who, at the time of the acceptance of the donation, are by law entitled to be supported by the donor. Without such reservation, the donation shall be reduced in petition of any person affected.
Civil Code, Article 750 — Giving Away Everything You Own. Read the full provision →
Giving away everything comes with a built-in condition
Article 750 allows a donor to give away all of his present property, but only under a specific condition: the donation may comprehend all the present property of the donor, or part thereof, provided he reserves, in full ownership or in usufruct, sufficient means for the support of himself, and of all relatives who, at the time of the acceptance of the donation, are by law entitled to be supported by the donor. If your grandfather is legally obligated to support your disabled uncle, that obligation had to be accounted for through a reservation of sufficient means before he could validly give away everything he owns.
What happens if no reservation was made
The article spells out the consequence of skipping this requirement: without such reservation, the donation shall be reduced in petition of any person affected. If your grandfather donated everything to your father without setting aside sufficient means to support your uncle, the donation is subject to reduction, not automatically void, but scaled back enough to ensure the support obligation to your uncle can actually be met. The reduction exists specifically to protect the person whose support the donor failed to provide for.
Who counts as a protected relative here
The protection under this article covers relatives who, at the time the donation was accepted, were by law entitled to be supported by the donor. Whether your uncle falls into this category depends on whether the law actually obligates your grandfather to support him, which typically turns on the specific family relationship and circumstances involved, such as your uncle's disability and dependency. If that legal obligation of support existed at the time your father accepted the donation, your uncle is exactly the kind of person Article 750 is meant to protect.
Who can ask for the reduction
The article allows reduction on the petition of any person affected, which is a broad standing provision rather than one limited only to the donor himself. If your uncle's right to support was left unprotected by the donation, he, or someone acting on his behalf given his disability, would generally be a person affected by the lack of reservation and could seek to have the donation reduced to the extent necessary to secure the support he is legally owed.