This idea has been brought up tons of times by grifters aiming to pivot from Crypto to AI. The reason that GPUs are used for blockchains is to compute large numbers or proofs - which are truly useless but still verifiable so they can be distributed and rewarded. The free GPU compute idea misses this crucial point, so the blockchain part is (still) useless unless your aim is to waste GPU compute instead.
IRL all you need is a simple platform to pay and schedule jobs on other’s GPUs.