"It may be doubted whether so small a number of men ever employed so short a space of time with greater or more lasting effects upon the history of the world.” — Sir George Otto Trevelyan on the Battles of Trenton and Princeton
I am pretty sure that lowering loyalty to 0 will not result in a sack but rather in making that city a neutral one (loyalty 1). The 5 plunder can only be claimed after successfully sieging a city.
This is not correct. Per 13.5,
A city reduced to zero loyalty will fall if there is a 0-value siege marker on the city. This check is made before replacing it with an independent city marker.
Thus, the Gauls have successfully besieged the city, and may choose between (a) making it a Gallic city with loyalty 1 or (b) plundering it per 14.4, claiming 5 plunder markers and making it a neutral city with loyalty 1.