Learn Swedish: Nu gäller det att njuta av ålderdomen i husbilen. - Now it is a matter of enjoying old age in the motorhome.

The phrase det gäller att is a very common idiom meaning "it is a matter of," "it is important to," or "the key is to." While the verb gälla literally means "to be valid" or "to apply," this fixed expression is the natural Swedish way to state that something is a necessary priority.

Why is it gäller det instead of det gäller?

This is a classic example of the V2 (verb-second) rule. Because the sentence starts with the adverb Nu (Now), the verb gäller must take the second position in the clause, pushing the formal subject det into the third position.

Since English often says "enjoy oneself", can I use a reflexive pronoun here and say njuta sig?

No, njuta is never a reflexive verb in Swedish. While you are currently learning about reflexive pronouns, you cannot attach one to njuta. To express "enjoying oneself," you would either use njuta on its own, or use a truly reflexive verb like roa sig (to amuse oneself) or the phrase ha roligt (to have fun).

Why is there an av after njuta? Doesn't njuta already mean "enjoy"?