Does iPhone need to be Unlocked GSM to work in Cuba?
As far as I can tell, any iPhone should be compatible with GSM. But the sticker on the back of the box is supposed to explicitly say what it's compatible with, so you can see that.

Note that phones provided by some carriers might support slightly different bands than phones from other carriers. I believe Cuba uses 900 MHz, and I would imagine all iPhones support that, but I did not check all models. But if there is a specific one you are going to get, you can check a website like WhistleOut to double-check that the phone supports this. You can google something like 'WhistleOut will my iPhone 8 work on another carrier', and the article will go over the variants of the phone coming from the different carriers and show what frequencies they support, so you can double-check.

If it's open box though, you do want to ensure that it will actually work, that it's not activated and associated with someone else's Apple ID, that kind of thing. If it's a fresh installation and it would work when you pop in your own SIM card (it does have to be the right size SIM card, probably a nano-SIM), then it should be fine for your sister too.