The GX was never intended to be a long term model. Initially it was supposed to only fill a couple year gap before a 3 row crossover replaced it. But it sold pretty well and has kept on selling 30k/yr or whatever it does. So I think it's been a success from the Toyota perspective.
I suspect the GX is going to face a tough road ahead to compete with the Sequoia. This is heavily dependent on Sequoia pricing though. I think the sequoia will be about $20k more than the GX for similar trims. So maybe the GX will still offer a lower budget alternative. (FWIW my guess is sequoia will range from $55-95k MSRP). If they do end up roughly in the same price range, I'm not sure how buyers would choose a GX over the Sequoia. Sequoia is sort of an LC300's big brother. Better power, better mpg, better luxury, better off road (where it fits). Other than size, and that's not usually a big issue in the USA, the GX becomes a pretty tough sell for now.
I think used GX seem like a great value. If I were buying in today's market, I'd look pretty hard at a used GX. I did the same when I bought my 4Runner. Drove them back to back a couple times and the LC200. 13mpg was really the killer for the LC for me. At that time used LC200's were roughly similar to new 4R. 4R was just better as an all around adventure platform for me at the time with more cargo space, more utilitarian interior that fit my needs better for hauling stuff and not people. I also have an RX350, so highway trips weren't as much of a concern. Now that I have kids, more towing, and less difficult trails, buying today - the GX would probably be my choice. I still like the 4R interior layout better and would rather just get a v8 4Runner.
Side thought: Now that Ford has announced that the American Ranger will get the 3.0 TTv6 with 400hp/430tq - is Toyota still going to be able to just offer the T24A-FTS in the taco and maybe next gen 4Runner? I'm not sure 275hp is going to cut it as a top engine. And... if Toyota has to put a TTv6 in the tacoma to stay competitive with the Tacoma, will there also be a TTv6 4Runner? Probably not. Not unless sales decline enough to force it.