Auto Fire rules apply to any weapon with a Rate of Fire of 2+, not just those with Rates of Fire of 3. This is confirmed by the first line of the Automatic Fire part of the Situational Combat Rules Section in the core Savage Worlds Deluxe rulebook. The applicable quote is
Automatic weapons (those with a RoF of 2 or higher) fire much faster - and inherently differently - than regular firearms.
Note that the original hardback version of the Deluxe rules had an error in this sentence, reading RoF of 3. This was corrected in a subsequent errata that can be found here.
This means that yes, all Gatling guns in Deadlands Reloaded support Auto Fire.