The son is losing out, poor mite. But the mum is losing too; if you have someone who consistently undermines a parent, the child's sense of security and 'rightness' in the world is damaged. They are taught not to respect their parent, and they can learn to manipulate one adult against another. It's the wrong sort of winning, no good for anyone and certainly not the child.
We don't know if that's the situation here, though there's an indication it might be by the OP's sentence that her mother undermines her all the time.
Not everyone in the world is nice, and there are manipulative people.
In the OP's shoes I'd be having a number of age-appropriate conversations over the years about 'people sometimes make mistakes and we should all respect a No. When you don't, it's not good behaviour'. It could apply to the granny, but it's a useful lesson in life generally.
The granny's the one who made the worst mistake here. She should have talked to the OP about the gift, and respected a No, even if she thought it was the wrong decision.
The OP was left with her mother manipulating her and her son (from what she said, not the first time) or with breaking her son's heart. That's a very nasty thing of the granny to have done.