Yes. My understanding is there will still be a fair amount of computation by "validators" who need to process transactions and run the methods on the smart contracts, but GPU mining will entirely stop at least on the ethereum network[1].

The difference between validator computation and computation by miners is that validators are running contract code that does something that someone has asked for vs miners are just running calculations just to prove they have (ie the calculations themselves are entirely unneccessary outside of mining.

[1] Bitcoin for example will still be doing this type of mining and "the merge" itself is entirely an ethereum thing.

Bitcoin hasn't been GPU mining in a very long time. It uses custom ASICs. The distinction is that bitcoin itself has very little effect on the GPU market. However, seeing as most coins are pretty correlated with bitcoin, at least until they completely fail, bitcoin price has like a second order effect on the GPU market.

Won't the impact simply be via ASIC miners instead?

no, eth is not changing the proof of work algorithm, it is moving to proof of stake.