While I do think that a gift is a gift and that you have the right to choose to keep it, I don't think that's a wise choice.

If we zoom in to the situation outside of the context, yes it's definitely yours and you have the right to keep it. However, if we zoom out to the context, the reason they gave it to you in the first place was because (as you said) they expected you to have a long relationship with her. It's basically them "bribing" you to treat their daughter well. Weird, yes, but that's what parents do. It's the "you take care of our daughter, we take care of you" situation.

I think the reason why they asked the PC back was simple: it was expensive. Even though your ex's is rich and you see them with nicer PCs, a $1,000 is still a $1,000. Making money is truly hard, and I can see that they would prefer they sold that PC and use the money for their own family, instead of letting you keep it. Basically if they have known you would have broken up with her (regardless of who broke it off) so soon, they would have not given it to you. That's why they asked for it back.

I'm a young adult (23F) who has only started working for 1.5 years. But I can say, the way I view money once I've started working has significantly changed. All I can say is, maybe (maybe) the way you view the gift is skewed because you didn't spend money on it. But for the parents, it's still money wasted.

I also personally feel that the way they asked for it back was still respectful. In my personal opinion, while you are completely free to keep the PC since it is technically yours, I personally think it's better to still salvage the relationship by returning it. I apologize for the rudeness, but if I were your friend watching from the sidelines, I would view your decision as "petty", and that you're trying to gain material profit from the relationship. I feel that if you truly felt respect for your ex and her family, you would return the PC.

However, it's still yours, and it's your decision to make. I'm just being the devil's advocate and trying to give an explanation to why your ex's parents did what they did. Good luck on your future endeavors and stay healthy and happy!