PDF of final rule (effective Jan 9) here:

a person may "possess, carry, and transport concealed, loaded, and operable firearms within a national park area in accordance with the laws of the state in which the national park area, or that portion thereof, is located, except as otherwise prohibited by applicable Federal law." 

Keep in mind this change will not allow carry in National Parks located in CA because even CCW holders are not allowed to carry in California state parks.

NRA-ILA :: Important Information On Right-to-Carry in Our National Parks.

the Department of Interior has released their new regulation that will recognize Right-to-Carry in national parks in those states that allow Right-to-Carry. It is important to note, however, that the new regulations do not take effect until 30 days after their publication in the Federal Register. The new rules, which were announced on December 5, were published in the Register on December 10. That makes January 9 the first day it will be legal to carry a loaded firearm in National Parks or refuges. Until then, the old regulations that ban possession of loaded firearms are still in effect.