Learn Spanish (Latin America): Viajar es caro, pero me gustan los viajes. - Traveling is expensive, but I like trips.

Because the thing you like is plural. Gustar works backward compared to English: it really means "to be pleasing to". Since los viajes (the trips) is plural, the verb must match it, becoming the plural gustan.

Why is there a los before viajes if the English translation just says "trips"?

In Spanish, when you talk about your general likes and dislikes, you must always include the definite article (el, la, los, or las). You need los viajes here, whereas English drops "the" and just says "trips".

The English translation says "traveling", so why does the Spanish use the infinitive viajar (to travel)?

When an action is the main subject of a sentence (like traveling is here), Spanish always uses the infinitive form of the verb. You cannot use words ending in -ando or -iendo as nouns the way we use "-ing" words in English.

Why is the adjective caro (expensive) in the masculine form here?

It describes the action of traveling (viajar). Whenever you use an infinitive verb as a noun, Spanish treats it as a masculine and singular thing. Therefore, the describing word must be the masculine singular caro.