genom is only a preposition. igenom can be used as a preposition, but it might sound a little more colloquial. As a verb particle, only igenom works.
- Jag går (i)genom skogen 'I am walking through the forest' – preposition – not stressed – both work but igenom more colloquial
- Jag går igenom mina papper 'I am going through my papers' – verb particle (or adverb, take your pick) – stressed – only igenom works.
English is notoriously bad for transcribing the sound of other languages, but yehnom as somebody suggested in this thread isn't a bad approximation. The g is soft (before vowels e, i, y, ä, ö) and pronounced like y in English yes. The e sound is a long, clear e as in maybe here in English.
Going "by" a restaurant and going "through" it are two entirely different things. "By" makes it sound like you're near the building, outside it, while "through" means you're inside the building crossing the interior. "Genom" means "by" in the sense of "by the means of something", so this sentence translates to "through" unless someone is somehow being transported somewhere by the restaurant.