The dies for the .270 Gibbs can be ordered from RCBS, Redding, or Hornady, but they are strictly a custom offering, meaning that they are not stocked and have to be made on a order only basis.
I know that the die makers are behind due to the increased demand that they are experiencing right now (12-16 weeks). I would contact Lonnie Hummel at Hornady, his E-mail is firstname.lastname@example.org
or call him at 1-800-338-3220 ext. 208
They will need 3 fired cases or a chamber reamer print to make the dies. They use EDM machineing and make the dies to fit the chamber of your rifle. If you haven't built the rifle yet, Pacific tool and gauge can send you a reamer print if they have it on file that Hornady can use to make the dies while the rifle is being made.
I'm not sure about RCBS or Redding but they may have the die reamers already on hand.
You are right about having to size up the brass to fire form a Gibbs. The Gibbs has a sharp shoulder an shorter neck than a "Ackley Improved" case with the shoulder being pushed forward. The resizing of the neck (running it through the Gibbs die), after having been "necked up" creates a false shoulder that the case can head space off of. You then Load this case seating the bullet into the lands. Using a mild Powder charge (a couple of grains off of max of a .270 Win load) you fire form to fit the chamber. You may have to do this twice if the cases don't fully blow out, but that's not usually the case. It's a similar method to forming the JDJ cases.
Hope this help