The gantry sits directly on the 20mm plates that will hold the rotary nuts, to which another 22mm thick plates with cars will be mounted. There's no gap there. The sides are 12mm thick steel. So there is direct contact of bottom of gantry ends to rail cars. There is no measurable side to side movement possible other than any play in the rails/cars.
I used 100x100x10 SHS for most of the construction as I had to order two 6m lengths and didn't want to throw away too much. Also one 6m length of 140x80x7 was used for gantry and for two table supports, all was used up almost without waste (supports intentionally placed along the table to create space for extra long 4th axis as we'd like to make some gunstocks in the future).
Adrian