posted 10-18-2018 12:07 PM               

A gantry is a a "bridgelike overhead structure with a platform supporting equipment such as a crane..." Gantries were/used to lift/erect the rocket onto the pad. As vehicles became more complex, the gantries were also used to provide access to various areas of the vehicle.

Also at the same time, as vehicles added upper stages and more advanced payloads, they required more services at upper levels. So umbilical towers became more massive than the simple poles used on early vehicles. It is these umbilical towers/masts that are erroneously called gantries.

Additionally with the use of off pad (VAB/VIF) or horizontal assembly, there is no gantry used on some vehicles.

posted 10-18-2018 12:49 PM               

Here is a 2011 an example of NASA using the term as such:

At the center of the circle stands the 247-foot-tall fixed service structure, or FSS, which is the gantry people see next to the shuttle.

posted 10-18-2018 10:09 PM               

It seems that as rockets grew larger in size, the gantry structure also grew and developed sub-systems such as walkways and access arms.

Both the Mercury and Gemini spacecraft had dedicated service gantries that were moved prior to launch, leaving the Launch Umbilical Towers behind to support the sequence. Perhaps the most well known image of these is this Gemini X Titan launch showing a time-lapse of the gantry retraction.

This image shows Alan Shepard's Mercury Redstone gantry behind the rocket, and the high tech cherry picker escape system to the lower right.

This image also shows the umbilical laying on the ground that can be seen falling away in this launch footage and also from this angle.

This article from The Atlantic has some incredible Mercury era photos, including an excellent view of many of the different launch complex structures.

By the time of Apollo/Saturn V, these gantries had morphed into huge structures, with the overhead cranes being installed in the huge VAB, and the service structure as a separate item.