From the introduction:
This document is a compilation of three different Gungrave jumps. They are all their own jumps, and can be taken at any point in your chain, however, the connection between these are a bit unusual. These jumps all contain an option to “link” the continuity for some or all of these jumps. Doing so not only gives your actions consequences across jumps, but links certain purchases together. Taking these purchases in one jump will also grant you any equivalents they may have in linked jumps, once you’ve visited them. Similarly, if you make a purchase with an equivalent from a linked jump you’ve visited already, you’ll receive all the versions from those jumps up-front.
You can take as many jumps between these as you’d like, but linking the jumps requires that you visit them sequentially.