Well, you're in PvE, so who cares how quickly you get it. Its not like you will lose it. None of the other pirates can actually pirate you, ironically.
Fastest way, join a bigger company, then steal their gold and leave. Or stay, who cares its PvE, not much difference. Should yield as much gold as you can get on a boat and sail away with. Don't skimp, you're on PvE so like I said, they cant fight you for it or steal it back. I recommend this one if your a solo.
2nd fastest way, get a sloop and some maps. Grab shipwrecks and flotsam on the way to the map. Do the map. Grab all the shipwrecks and flotsam on the way back. Again, if you even bother going back. Its PvE, so you can literally live on your boat. Should pull down 1000-3000/hr depending on maps and luck. Id recommend this one if you are a small group, 3 or less.
Slowest way, go through the trouble of building farmhouses and WHs and a market and get that all going. Earn 30-5000 gold a day. Id do this one if you have say 5 or more people to help out. (Also do the 2nd one as well.)