It is no longer true, there are many miners that allow GPU mining using argon2d. More explanation here: https://www.reddit.com/r/garlicoin/comments/7wf3m6/argon2d_is_gpu_resistant/dyrir63/?utm_source=reddit&utm_medium=web2x&context=3

Currently, the most GPU/ASIC resistant functions are probably yescrypt-based. But in reality, there is no such things like "ASIC-resistance". People are not building ASIC machines for such algorithms, because now it is not profitable yet. But when some altcoins will be worth more, they will be created sooner or later. And the only defense in all such cases is changing an algorithm, but it possibly leads to creating a new coin.

GPU resistance is regularly broken on many algorithms, because it is easier to get some GPU hardware and write some code than create some ASIC device from scratch.