Pokémon Soulgold walkthrough
10 parts, one per checkpoint Pokémon Soulgold’s own documentation defines: the areas each one opens, the wild Pokémon and trainers in them, and the fight at the end of it. Everything here is read off the game’s own files, so anything it doesn’t state isn’t claimed.
Part 1: New Bark Town to Violet City — Falkner
14 areas · 26 trainers · Crystal, Gold, Falkner 1 and 3 more
Part 2: Union Cave and Azalea Town — Bugsy
7 areas · 27 trainers · Proton 1, Bugsy 1, Bugsy 1 (Hard) and 3 more
Part 3: Ilex Forest and Goldenrod City — Whitney
16 areas · 63 trainers · Gold, Crystal, Whitney 1 and 17 more
Part 4: National Park to Ecruteak City — Morty
11 areas · 42 trainers · Silver 3 (Chikorita), Silver 3 (Cyndaquil), Silver 3 (Totodile) and 4 more
Part 5: Olivine City and Cianwood City — Jasmine and Chuck
21 areas · 61 trainers · Eusine, Chuck (4 badges), Chuck (4 badges, Hard) and 16 more
Part 6: Mahogany Town and the Lake of Rage — Pryce
19 areas · 56 trainers · Pryce (4 badges), Pryce (4 badges, Hard), Pryce (5 badges) and 5 more
Part 7: Goldenrod Radio Tower — the Rocket takeover
3 areas · 15 trainers · Petrel 1, Ariana 1, Petrel 1 (Hard)
Part 8: Blackthorn City and the Ice Path — Clair
11 areas · 27 trainers · Crystal, Gold, Clair 1 and 5 more
Part 9: Victory Road and the Indigo Plateau
13 areas · 38 trainers · Silver 5 (Chikorita), Silver 5 (Cyndaquil), Silver 5 (Totodile) and 22 more
Part 10: Post-game — the wider world
56 areas · 78 trainers · Crystal, Gold, Crystal (Hard) and 31 more
It also doesn't say when Dark Cave North Side opens up, so it is in no part above; it has its own route page.