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
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.