grammaticality - Is “gaggle of girls” plural or singular, or does it depend?

How can I tell whether constructions like “X of Ys” should be considered singular or plural, given that X is singular but Y is plural?

1. A gaggle of girls boards the train.



2. A gaggle of girls board the train.




3. The gaggle of girls was running to catch the train.



4. The gaggle of girls were running to catch the train.

Can one version be right sometimes but the other right other times?

Can both versions ever be right?