A gerund is a noun formed from a verb. To make a gerund, you add "-ing" to the verb, just as with a present participle. The fundamental difference is that a gerund is a noun, while a participle is an adjective:

gerund

I enjoy running. ("Running" is a noun acting as the direct object of the verb "enjoy.")

participle

Stay away from running water. ("Running" is an adjective modifying the noun "water".)

Rumpus room is a construction in which a noun (rumpus) is serving as an adjective to modify the noun "room" exactly as in -- for example -- "coal mine" and "dog house". In the expression "dog collar" is an adjective.

All clear?

A