I was gifted an item but it says I don't own it?

Yeah I believe it has something to do with trying to gift directly from someone's wish list vs. going to the store page and gifting like that as I had a friend try it from my WL page, but even thought it showed up showing that I had been gifted something, it came up with the same thing as you got that I needed to buy it and it also didn't show up on my friend's purchase history. So she tried it by going to the actual page of the item and did the gifting option from there and it worked that time.

I really wish that they would fix the store site of the glitches that it gets but I'm afraid that they're not going to waste time trying to figure out what's wrong as they still haven't fixed the Weekly Sales page since it's still stuck on the October 2016 sales.

Here's a link to a thread about gift problems from up in the gift gathering part of the forum:

https://forums.thesims.com/en_us/discussion/938358/not-getting-gifts-from-other-people#latest

I haven't had any problems with it, so can't offer any personal advice, but looks like there wasn't one definitive answer. Sometimes it looks as though going via a different route (via the gifting email as opposed to directly to the store page, for instance) or having the gift giver try again works out. I would guess re-gifting is assuming they also didn't get charged for it the first time. Good luck - hope you get it sorted out.

Ok I will contact the gifter and let them know.I don't think they're gonna update it anymore, isn't it technically closed?

They still do maintenance on the site every now and then. But I believe that's about it though.