Let's take an in-depth look at gerunds, present participles and the difference between them:
Gerunds are words that are formed with verbs but act as nouns
Present participles do not act as nouns. Instead, they act as modifiers or complete progressive verbs.
Thus, gerunds will be subjects, subject complements, direct objects, indirect objects, and objects of prepositions. Present participles, on the other hand, are complete progressive verbs(for example in present continuous, "he's swimming" or act as modifiers (as in, swimming pool).
So, I guess we can agree that "swimming" in "swimming pool" modifies the noun "pool", I.E. it's a modifier and since gerunds can't be modifiers, "swimming" has to be a present participle (by process of elimination!)