Learn Italian: Chiunque può chiedere aiuto al custode. - Anyone can ask the keeper for help.

In English, you "ask [someone] for [something]". In Italian, the verb chiedere uses the structure chiedere [something] a [someone] (literally, "to ask something to someone"). That is why "help" (aiuto) comes right after the verb, followed by al custode (to the keeper).

Is chiunque always paired with a singular verb like può?

Yes. Chiunque (anyone, anybody, whoever) is an invariable indefinite pronoun. Just like in English, it always takes a third-person singular verb, no matter how many people it might refer to in practice.

How do we know the gender of custode since it ends in an "-e"?

Nouns ending in -e can be masculine or feminine. Custode happens to be a noun whose ending doesn't change based on gender, so it can refer to a man (il custode) or a woman (la custode). Here, we know the keeper is masculine because of the articulated preposition al (a + il). If it were a female keeper, it would be alla custode.