It depends. If the portrayal of the yandere girlfriend is done in an interesting and nuanced way, it could be appealing. But if it's overly stereotypical or one-dimensional, then no.
A gift from her in a yandere manga can have multiple meanings. It could be a sign of the yandere's obsessive love, an attempt to control the situation, or a way to create drama and tension in the story. Sometimes, it might even be a red herring to mislead the reader.
Well, such a girl is often obsessive and possessive. She might go to extreme lengths to keep the person she likes close to her, sometimes even using violence or threats.