Is adobestock approves Photoshop upscaled images ?

I should mention, what I like best is that Gigapixel creates a new layer with the upscale. If something gets overly sharpened,, like hair, irises, fingernails, bushes, I mask out those areas to reveal the original. It might just be my imagination, but the original is also sharpened, but to a lesser degree.

Upscaling is a mixed bag of tricks. If you're too heavy-handed with it, you'll likely make your images worse, not better.

Start with a high resolution product.  Don't attempt to upscale small, low-resolution images. In the absence of usable pixel data, upscaling just introduces unwanted noise & artifacts.

Experiment and carefully examine results at 100-300% magnification. 

If the upscaled results aren't good, discard and try again at lower settings.

Rinse, lather, repeat.

Yes. The best upscaler is Gigapixel AI, but free upscalers are available on the internet. But bear in mind that all upscalers can introduce and will magnify issues such as grain and noise.