A while ago, I bought Topaz Gigapixel AI, because my feverish fantasies led me to believe that it would allow me to turn heavily cropped photos into multi-megapixel masterpieces. I mean, it says AI on the tin, which is basically a shorthand for magic and miracle, right? Well, the tool does a decent job of upscaling photos. But you know what? It turned out that RawTherapee is no slouch in this area either. The only difference is that the latter doesn't rely on multi-gigabyte models, and the application is free in every sense of the word.
To be fair, in certain situations Topaz Gigapixel AI can produce pretty decent results, but most of the time the output is only marginally better than photos upscaled with RawTherapee. And I'm not so sure whether the difference is worth is $99 (plus, whatever you have to pay for upgrades). The screenshot below is definitely not the best way to demonstrate the differences (or the lack thereof) between two photos upscaled with both applications, but it can give you a rough idea of what I'm talking about:
Both photos where upscaled with 2x factor, with all other corrections turned off. Topaz Gigapixel AI tends to produce smaller and slightly sharper JPG files than RawTherapee, but you really need to look closer to notice any discernible difference between the two upscaled photos.
You can download the two upscaled files for your pixel-peeping pleasure, if you like.
Topaz Gigapixel AI is not bad. But I'd expect more than "not bad" from a tool that 1) costs money, 2) is essentially a one-trick pony, 3) uses the most advanced technology available to perform its trick. If it can't outperform applications like RawTherapee by a meaningful margin, then what's the point?