Below is the 4th meaning and example sentence for the verb "tell", from Oxford Advanced Learner's Dictionary.
- [intransitive] (informal) to let somebody know a secret
e.g.) 'Who are you going out with tonight?' 'That would be telling!'(=it's a secret)
Is "telling" in bold a gerund, or just a present participle that forms a present-continuous form along with "be" in front of it?