If a gentile does Shabbat should he die?

Yesterday I read someone here quoting Ramban’s texts. And I saw that he said that non Jewish people who try to do Shabbat or other things like trying to study the Torah should be brought to a Jewish court and get killed.

Is this true or I am misunderstanding something?

It sounds so agressive to me. Why would a Jewish forbid their traditions to non Jewish people. If I feel curious and drafted into jewish culture and theology why cant I read or practice their traditions and texts. They keep the words of g-d and its prophets, everything that is good should be share with those who keep a good heart and respect.

I am agnostic and recently I am reading a lot about Judaism. Last Saturday, I was feeling really tired so I (as a freelancer) decided not to work, and felt myself as I will honor G-d and jewish culture by resting on Saturday and do my own personal Shabbat. I know it’s a silly idea, Im not jewish and I dont know enough to even do a proper Shabbat, but now I see that just feeling involved in the idea of Shabbat is deeply wrong and I should be killed in Rambans perspective.