Nowadays, with so many high-rise buildings, neighbors rarely interact with each other. Neighborhood atmosphere and relationships like yours are really rare and worth cherishing. You should maintain them well. However, for such family gatherings, you should arrive earlier. There are not so many rules. At least you should help out a little. But it should be appropriate. After all, it is not hosted by your family. Don't steal the show.
It is recommended that you bring a bouquet of flowers or a bottle of good wine. Flowers can liven up the atmosphere, and the fragrance will also make the participants feel better. However, if you want to bring a gift, you can bring a simple and practical home decoration. You can order a colorful fluorescent pillow. It will also enhance the color of the host's sofa. Haha, I hope it will be helpful.