The most efficient way to grind gold?
grinding isn’t efficient, playing the market is. if you insist on grinding, do HoT meta’s and silverwaste metas/high level fractals.
When I want to do a mindless grinding, I usually collect some materials from (almost) all the nodes in Bloodtide Coast & Sparkfly Fen. Selling those material in TP will get you more than 15 Gold, with some more materials to spare.
play WvW or ktrain EOTM
SW and Dungeons are among the top tier and stand toe to toe right now. SW is self explanatory, you run around and tag every fort, then do breach, VW. Can chest train too.
Dungeons you do your daily eight. I recommend all of AC, CoF, then do SE P1 and P2. At the end transfer the 150 tokens you get from your 5 gold achievement, to a high level dungeon. Use all 150 to buy five rare helms to salvage for ecto. Use all of your CoF tokens to buy rare helms too. AC and SE save for exotic armors to salvage for insignia. Depending on your speed you could surpass SW income.
Fractals apparently give good gold too, but I disqualified them due to the cost to purchase the agony resistance.
A little less gold but more relaxing, is to go on gathering routes.
Because of how the market operates, I can’t share my actual farming location, but if my location were to ever be nerfed, I’d personally spend most of my time in dungeons because I know them very well.