Good man, you are making me justify it, but I do think you are making the maths slightly complicated...

The 1.5" diameter tube has a circumference of 120 mm, same as the PD of the pulley. So in 1 turn it moves the same distance as the gantry. I can simply add the tube weight to the gantry weight.
OTOH, point taken, 16 gauge can handle 0.66 Nm of torque, I can save myself 1.4kg

Let's up the gantry weight estimate from 20 to 25 kg, required torque goes up in proportion 0.44 Nm becomes 0.55 Nm. To get the safety margin I either need a bigger motor or a lower acceleration.