Doesnt the Gadhimai massacre of countless animals in Nepal violate the principle of Ahimsa? Can it really be called a 'Hindu' ritual?
Yes, it does violate the principle of ahimsa. This is not for those who are interested in self realization.
Below are couple of verses from Srimad Bhagavatam spoken by Narada Muni:
7.15.10 Upon seeing the person engaged in performing the sacrifice, animals meant to be sacrificed are extremely afraid, thinking, “This merciless performer of sacrifices, being ignorant of the purpose of sacrifice and being most satisfied by killing others, will surely kill us.”
4.28.26 That most unkind king, Purañjana, had killed many animals in various sacrifices. Now, taking advantage of this opportunity, all these animals began to pierce him with their horns. It was as though he were being cut to pieces by axes.
And Srila Prabhupada writes in the purport:
Those who are very enthusiastic about killing animals in the name of religion or for food must await similar punishment after death. The word mamsa, “meat,” indicates that those animals whom we kill will be given an opportunity to kill us. Although in actuality no living entity is killed, the pains of being pierced by the horns of animals will be experienced after death. Not knowing this, rascals unhesitatingly go on killing poor animals. So-called human civilization has opened many slaughterhouses for animals in the name of religion or food. Those who are a little religious kill animals in temples, mosques or synagogues, and those who are more fallen maintain various slaughterhouses. Just as in civilized human society the law is a life for a life, no living entity can encroach upon another living entity as far as the Supreme Lord is concerned. Everyone should be given freedom to live at the cost of the supreme father, and animal-killing — either for religion or for food — is always condemned by the Supreme Personality of Godhead.
However we cannot deny the authority of the Vedas which allow the sacrifice in a restricted manner for gradual advancement.
As for the Nepal event, such a mass slaughter definitely violates the idea of doing it in a 'restricted' manner so my take is it must be a concocted ritual.
Animal sacrifice is spoken of in the vedas. It belongs to another era. People that do animal sacrifices are, like people that do other types of sacrifice, trying to gain some worldly or heavenly (not brahmaloka) merit.
Animal sacrifice is animal sacrifice - no matter what the actual number of animals that are involved. Goat sacrifices are still done by Brahmin priests at the Kalighat in Kolkata. It is a Shakti Peetha.
If an animal sacrifice is done according to vedic rules by a Brahmin priest, then the results will be according to the vedas.
As a general rule for people, killing outside of the rules of the vedas is a sin.
There are no vedic sacrifices of animals that lead to brahmaloka.