This is what I see why the price of grin coin drop by half because someone already mines grin in early months where the difficulty is too low and get benefits of selling them all these days that cause the price drop.
How about after obelisk miner release? This will be a big problem if they release this miner and I am sure GPU mining will be useless with this coin.
GPU mining will eventually become useless not because of Obelisk but because of built-in gradual deprecation of the C29 algo. On the other hand, for the same reason Obelisk (or another ASIC) won't affect GPU mining much if at all. ASICs and GPUs don't really compete directly.