I might as well just remove it in the next version from the Gauss Rifle
I believe it's about the actual ammo the Gauss Rifle uses.
The weapon, in Fallout 2, used 2mm ammo and no MF Cells. In Fallout 3, the weapon was introduced later and Beth probably thought that there was no reason to make a whole new ammo type for just one new gun. So they added the microfusion cell thing (which IMO is realistic for a coilgun) and intended to have the 2mm ammos in a backpack. However, since this would seem silly especially for snipers using the weapon, they removed the backpack (or never made it) but forgot to uncheck the "hide backpack" thing.