I found an example of using "the" before the gerund:
During the crossing the passengers felt the ship toss
But according to the grammar, no article is used before the gerund. Or in this case the crossing isn't the gerund?
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Gerunds act like any other noun and can take the definite or indefinite article when referring to a specific instance of an activity:
a reading of my latest book
the singing coming from the church
The rule you’re citing is that the gerund does not take the definite article when referring to an activity in general, but this rule also applies in other cases where the noun isn’t a gerund:
I like swimming.
I like basketball.
I suspect this rule is taught because it’s explicitly different from many Romance languages, where the definite article is used to refer to general activities.
'Ing' structure can be used as a noun.
If you want more information, check this link
Here is an example from Cambridge Dictionary.
On page https://dictionary.cambridge.org/dictionary/english/tick you can see "the ticking" in the definition of verb "tick".
The ticking of the clock kept her awake.