It's really not difficult. There's two-three main sources of money:
- Follower missions. Since you qualify for any missions, regardless of level, that reward gold, I have 2 to 3 gold missions waiting for me on average, whenever I log a character. To those missions you can add an additional mission from a level 3 Inn. You get anything from 100 to 400gold waiting for you. I do that twice a day (morning before work, evening coming back).
- Salvage. A level 3 Salvage Yard means that every level 100 mission and 20% of level 95-99 missions yield a crate of salvage that has 1 grey item that sells from 50s, a few ressources, a green that sells from 5 to 25g, and a follower token that you can also sell for 1-2g. I usually launch 6-7 level 100 missions every time, meaning I get an additional 100g or so in crates twice a day.
- Selling any resources you don't use at the AH (even at bottom prices if you're not an AH player). You can use the trade post to convert extra GR into more ressources.
Basically, with 15mn a day (launch missions, harvest mine & garden, open crates), you get you 800-1k gold without leaving your garrison. All you need is a level 3 barracks (for more followers for more missions in parallel), a level 3 inn (10 dungeon quests to do to unlock), a level 3 salvage, and any trade post. Recruit Treasure Hunters every week at the Inn for extra gold and you get about 50% more on average. Multiply by the number of alts you're willing to devote your time to.