The Gerundivum is an adjective derived from a verb, and has a passive meaning. You can have it in singular or pluaral and in all cases (a or o declension). There are different way in which you can use it, seems like this is where you're confused, so here some examples:
you can use it attributive:
Uno anno duo consules sine memorando proelio ab Hannibalis militibus iterfecti sunt.
In one single jear two consuls were killed by Hannibals soldiers without a battle worth remembering.
Hannibal insidias parandas curavit.
Hannibal was responisble for the carrying out of the attacks (I'm no good at translating exactly, I know
or as a noun belonging to the main verb:
Denique bellum in Africa finiendum erat.
At last the war had to be ended in Africa.
Don't be mixed up with the Gerundium and Gerundivum.
Hope that helped a bit.