Strictly speaking, there's no 64 bit version of GSX.
We recently updated the *Couatl* scripting engine to be 64 bit and a native client for P3D4, compared to the previous version which was used by all simulators, which was a 32 bit FSX Simconnect app, which worked in P3D4/5 because of the backward compatibility P3D has with the FSX Simconnect.
The advantage of using the 64 bit version is that it should be less dependent on different Simconnect versions, and it's not dependent on older VC++ runtimes like the 32 bit version is.
As far as GSX is concerned, the main visible difference is that P3D4/5 will use a different set of vehicles, made in PBR with much higher quality. If you were already using P3D3.4 and P3D4.5, you surely already have the PBR objects installed, because the FSDT Live Update would have installed them anyway, if you ran it at least once in the past year, after the PBR update came out.
The 64 bit upgrade is very recent and, again, it's done entirely by the Live Update. If you have P3D4 ( or 5 ) installed, running the Live Update will enable the 64 bit version to be active in the 64 bit simulators so, there's nothing for you to do, and nothing to reinstall.