of course you can estimate weight of gantry

cross-section of 100mm x 3mm box section is 100*100-94*94=1164mm^2
gantry is 1350mm wide approx. so total volume of steel is 2 * 1350 * 1164 = 3142800mm^3 = 0.0031428m^3 @8750kg/m^3 = 27.5kg
add spindle, carriages, energy chain, y & z stepper, ballscrew/rack, y-rails, z-rails, etc. (all of which you can get from data sheets or calculate), and your gantry is in the order of 50kg which is typical. add 30% safety margin and for sizing purposes use say, 65kg. - its good enough to help decide what motor is needed.