No and yes. No, it should not be gentil, the -s is correct. And yes, on is 3rd person singular. But (and there always is a but) when on stands in for nous, it is used in a plural sense. So you have a chimera demanding a singular verb (est) and a plural adjective.

In the case that on is the impersonal "we", i.e., one/people, the adjective is singular.

Chris - I think you meant a plural adjective, not adverb.

Merci Chris et Maarten