We say a gentile that keeps Shabbos is chayav misa (death penalty). Is this also true if a gentile keeps yom tov?
Keeping Shabbat is not necessarily Shabbat - the Gemara (Sanherin 58:2) states it can be on any other day as well.
Resting for a non-Jew is our human intuitive definition for rest (as I understand from this source), not keeping Halachic Shabbat.
Apparently, there would be no difference if it's a Yom Tov or Shabbat - he did שביתה at any rate.
The Rambam (Melachim 10:11) says
וכן גוי ששבת אפילו ביום מימות החול אם עשה אותו לעצמו כמו שבת, חייב מיתה, ואין צריך לומר אם עשה מועד לעצמו.. He continues to say they are not allowed to make up new Mitzvot - so the problem is doing it as a Mitzvah.
On a similar note, Radbaz asks what the problem is if it's on a different day and explains the problem is declaring a certain day of the week as a day of rest, not one-time resting.
I would conclude he has the same חייב מיתה (I've seen places emphasize: this doesn't mean Beis Din will kill him).
Some sources explain this as a violation of the special connection between Shabbat and Yisrael. This can be argued pro or con.