It say's other armor applies. Does that mean it stacks or not?

jh

In particular it says: "You may wear armour over the gutplate; if you do that armour's soak and defence apply."

Since it says "apply" and not "add up" or similar, the values of soak and defence of both armours does not stack. You simply use the soak and defence values of the armour which is "on top" of the gutplate. Note though, that it does not say anything about the special ability of the gutplate, which you can still use. If you have an ogre with a gutplate and a fullplate armour, you have effectively an ogre with a gromril armour, (I say that because I know you love dwarfs and specially Ironbreakers )