Using the Collatinus conjugator there isn't a form for plural gerund, but there is for the gerundive since the gerundive acts like an adjective. I therefore always assumed that gerunds are never plural. However, I'm looking at the Lasla database and they have many words in the gerund plural unless it is a mistake. It should be noted however that all of the plural are in the dative or ablative case, never accusative or genitive, though so far I've only looked through 40% of the database. If you need me to track down the actual text that these words come from then I can do that but I'd rather not since it will take quite a while for me to do that.
Also, the Greenough grammar just says:
The gerund expresses an action of the verb in the form of a verbal noun. As a noun the gerund is itself governed by other words; as a verb it may take an object in the proper case.
I would think that because they do not specifically state that the gerund cannot be plural then it would follow that it can since nouns without plural are the exception rather than the rule.
Also on sec 190 of the Greenough grammar 1916 edition, they just list the first form of the gerund and expect you to fill in the rest.