No. Many times they have different tube sizes so be careful when you make that comparisons.
The gatsby and the sierra aren't the same they just use the same bushings - the gatsby is a finial twist where the sierra is a section twist
JT is correct in more than one way regarding tube sizes. It isn't just length. There may be very subtle differences in diameter, even though the tubes are "spec'd" the same.
Rockler's Manhattan is a section twist, like the Sierra. Rockler's Gatsby is a finial twist. It may be the same as Gatsby kits sold by others. I bought some of both types of kits from Rockler. I also bought spare tubes at Rockler. They have separate part numbers. The Manhattan tubes come in a 5-pack, while the Gatsby tubes come in a 2-pack. Rockler also sells bushings as separate parts for Manhattan and Gatsby.
(Edit added later: The bushings are the same size.)
I eyeballed the tubes by holding them side-by-side between my fingers. They looked and felt identical in length and diameter, so I tossed them in a bag together. Eventually I had some kind of turning failure with one of the kits, so I pulled out a spare tube and a new blank and started over.
It was during assembly that I discovered that the pen parts could be easily pressed in by hand. The new tube had an ever-so-slightly larger internal diameter than the other type. I tried other tubes in the bag, and sure enough, there were two types. A drop of CA glue was all that was needed to fix the issue for me, but now I know.
I am convinced that the pen tubes were intended to be the same, and the slight variation is a just a manufacturing difference between the two different factories.
