mass has nothing to do with dampening and dissipating vibration energy.
it lowers the resonant frequency of the machine, or increases it, and for a given impact the increased mass lowers the resultant velocity of the impacted device, which lowers the amount of audible sound created. but the energy still has to go somewhere.
The hollow cavity inside a gantry is mostly useless for adding stiffness, because you can only add stiffness according to its share of moment of inertia, which follows the outside dimension of the structure to the 4th power.
if your cavity is only 3/4" thick x 6" x 36" then i don't think you're going to make any difference regardless what you fill that cavity with. if you meant to say your gantry is 3/4" thick leaving a 6x6" cavity inside, 36 inches long, then you would have a great chance of dampening vibrations. just fill it with a mix of lead shot and sand.
basically the figure of merit for machine dampening is modulus of elasticity multiplied by dampening coefficient. loose materials which have no stiffness but have incredibly high dampening coefficients dissipate energy by friction between the particles, rather than in the material.
silicone rubber has great dampening coefficient but low modulus of elasticity.
if you have an outside surface or two of your gantry free, you could take a tube of silicone or butyl rubber (not sure which is better) and coat the outside surface of the gantry with strips of it like this |||||| leaving air spaces between them so the chemical can cure. use some spacers to control how much the goop gets squished out, and set another sheet of 1/4" by 8" by 48" long steel up against the goop, compressing it slightly but still leaving air gaps between the lines of goop.
if it doesn't work, you can cut the metal off with a knife/saw through the goop and cut the steel away and try something else.
two part polyurethane foam is also good at absorbing vibration but its stiffness is relatively low. you might fill your void with 16 pound per cubic foot polyurethane foam.. it only costs 4$ a pound..