I am playing a solo game and the non Player Gaul can throw a dice to check if he will play the mentioned event.
But I want to ask a thing first, there are not 8 Warbands in available (there are already 9 placed on board), only 6.
Sinve the event want 4 Warbands each in Nervii and Treveri, is it considered ineffective?
No. Per 5.1.3 you execute as much of the Event as possible: place 6 Warbands, then execute a Germans Phase minus the Rally.
Even if there hadn't been a single Available German Warband (i.e., if all 15 German Warbands had been on the map already), the Germans Phase part of the card would have an effect.