There in no free online version of the Gaceta Oficial. For a susbcriber-based legal research service on Dominican law, go to Lexlata (
www.lexlata.com). (I'm a cofounder and Director of Lexlata.)
All presidential decrees, including the ones regarding naturalizations, are published in the Gaceta Judicial.
I'm unable at this time to answer your question regarding how the time for naturalization is computed because I'm presently out of the country with to time to access the statute, etc.