---Updated In Light Of Recent Comments---
Generally speaking it will depend on your company's policies. Not so generally speaking, however, if a manager walks up to you and claims that the laptop belongs to the company then you have to decide whether hanging on to it is worth your job or not.
Conference swag is, in my experience, each participant's to keep. I've never heard of the company demanding that their employees hand over their t-shirts, pens, or other such trinkets. That being said, a laptop is worth quite a bit more than a t-shirt.
On the Wikipedia page it lists every such convention that has taken place, and it mentions that in 2015:
- The ticket cost $2095 USD
- "Build attendees received a free HP Spectre x360 ultrabook"
Some managers may feel that since the company paid for the (pricey) ticket they are entitled to the "gift" - let's be serious, it was not so much a gift as a part of the package. This may especially be true if it's a smaller company and the price of a laptop is actually a significant expense to them.
Seeing how they've known about this machine yet allowed you to keep it for personal use makes the situation confusing. Had they immediately established that it was a work machine, that would have been fine (and I highly encourage anyone in a similar situation to hash this out with their employer as soon as they receive such a generous gift).
However, a year of personal use later I can see how you'd be upset at parting with it.
At the end of the say the most you can do is state your case:
With respect, I received this laptop as part of my convention gift package, and have been using it as a personal machine for almost a year. It's a little late in the game for you to claim it as a company resource.
That may (most likely will) not go over well. At that point you could simply quit and keep the laptop and they probably won't take legal action against you. Probably.