Any GPU with high VRAM (I think you need at least 8 GB to be efficient) can mine Grin, but at a loss.
E.G. 3090Ti woul have daily revenue of $1.23, and a profit of $-0.98 at electricity costs of 0.1$ per kWh.
So in reality no one would mine Grin on GPU unless they want to perform a costly attack using GPU power and even then we have grin defender which will rent GPU power to counter the attack or at least make the attack even more costly.
According to WhatToMine AMD 6800 GPU and similar, are breakeven at around $0.10. But I have no confirmation from the community on AMD 6800 Mining results. The next Generation AMD RX 7900 XT(X) might be even more efficient or not. I have not seen data about that. The interesting thing is, if we would see a network hash-rate inflation of for example factor 2 to 10 we could [edit: not] know what kind of hardware is used.
An other reason to use GPU might be privacy, you can not buy Mining ASICs without loosing privacy. But general purpose hardware like GPU have more sellers, more other reasons to buy, or you have it already. So this privacy leak is smaller.
Understood, the reason we did not prioritize the GPU as grin easily got attacked. However, maybe in future when we’re mature enough, we would consider to ‘re-open’ for GPU mining.
Without further ASIC development it seems like GPUs might catch up in a few years.