That's because there's no such thing as a "sentience-o-meter", and there's no need to "operationalize" or "measure" anything. Instead, what's wrong with the definition given by Wikipedia, "ability to experience feelings and sensations"? That surely aligns with about 2500 years of philosophy and common sense, preceding all the techno mumbo-jumbo that confuses us today.
Sentience is a property of the higher forms of life, i.e. animals, which is derived from Latin "anima", meaning "soul" or "spirit".¹ Mammals and birds qualify because we relate to them easily and naturally.
Artefacts like GPUs don't even have metabolism, they can't procreate, they're dead matter, and having electricity running through them in intricate circuits doesn't change that.
[1] Some languages make grammatical distinctions based on whether an object is considered "spirited" or not, surfacing fundamentals of human perception of the world at the level of grammar.