I used to swim in the swimming pool. Here ‘swimming’ is a-

Sometimes it is difficult to distinguish between a present participle and a gerund, since gerunds can also be used as adjectives.
For example:
In the phrase 'swimming pool' the word 'swimming' is a gerund used as an adjective.
- We know this because, swimming pool means pool that is used for the activity of swimming.
- It is not a present participle because the pool is not doing the action of swimming.

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Lets see another example:
- He dived into the swimming pool.
- He waved to the swimming children.

In the first sentence, the pool is used for swimming. So it is a Gerund.
In the second sentence, children are doing the action of swimming. So it is a Present Participle.

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Here are some other examples of gerunds used as adjectives:
- Sewing machine
- Frying pan
- Baking sheet
- Fishing rod
- Bathing suit
- Drinking water
- Shaving cream
- Ironing board

Source: Gmat Club and Applied English Grammar and Composition, P.C Das