Learn Hardcore Indonesian: Rombongan menunggu di gerbang sebelum perjalanan dimulai. - The group waits at the gate before the journey begins.

Rombongan literally comes from the root rombong plus the suffix -an. It refers to a group of people travelling or moving together with a shared purpose—often on a trip or excursion.
By contrast, kelompok is a more general word for “group” or “cluster” and can be any assortment of items or people (e.g. a study group, a discussion circle). Rombongan has the nuance of “a party” or “a traveling party.”

Why is the verb menunggu formed with the prefix me-? How does the sound change from tunggu to menunggu?

Indonesian uses the me- prefix to form active-transitive verbs from roots. The root tunggu (“wait”) takes me-, but because t is a voiceless stop, it undergoes nasal assimilation and becomes men-, yielding menunggu.
Rule of thumb: me- + p/f → mem-; me- + t → men-; me- + k → meng-; me- + s → meny-; me- before vowels stays me-.

What is the function of di in di gerbang? Why isn’t there an article like “the” or “a”?

In Indonesian, di is a preposition marking location (similar to “at,” “in,” or “on”). There are no definite or indefinite articles (a, the) in Indonesian—di gerbang simply means “at the gate,” and context determines if it’s specific or general.

What role does sebelum play in this sentence? Can it be used with both nouns and verbs?

Sebelum means “before” and introduces a time clause. It can be followed by a noun phrase (sebelum rapat dimulai – before the meeting starts) or a full verb phrase when the verb is marked (e.g. sebelum dia pergi – before he/she leaves). Here, sebelum perjalanan dimulai is a subordinate clause “before the trip starts.”

Why is perjalanan used here? What is its root, and what exactly does it mean?

Perjalanan is a noun meaning “journey,” “trip,” or “travel.” It’s built from the root jalan (“to walk” or “road”) with the prefix per- and suffix -an, turning it into an abstract noun for the act or process of traveling.

Why is dimulai used instead of just mulai? What does the di- prefix signify?

Mulai can be both an adjective (“starting”) and an intransitive verb (“to start”). When you add di- to form dimulai, you create a passive verb: “(it) is started/has started.”
In this sentence, perjalanan is the grammatical subject receiving the action: “the trip is started.” Implicitly, the group initiates it, but grammatically it’s passive.

Could we reorder the clauses without changing the meaning? For example: Sebelum perjalanan dimulai, rombongan menunggu di gerbang.

Yes. Indonesian allows you to front the time clause for emphasis or style. Both versions are equally correct and convey the same sequence: first the waiting, then the start of the trip.

If I wanted to express that the group had already been waiting, how would I add that nuance?

You could insert sudah (already) after the subject:
Rombongan sudah menunggu di gerbang sebelum perjalanan dimulai.
This implies the waiting was completed or ongoing prior to the trip’s start.