Nothing is 100% GPU (or even ASIC) resistant. It's just a matter of effort and money whether someone will succeed in creating a GPU miner.

Our approach with ZETTEL and its constantly evolving "Burgerhash" algorithm is that every 100k blocks (ca 10 weeks) we are increasing the complexity of the POW algorithm,
so that any hypothetical GPU-miners (should they come into existence) become obsolete.