Grand Ages: Medieval

The best way to gain gold is trading, have some proper trading routes set up and money will be coming in on its own. 50 million is a lot and it will take a while, when you spend money on items (businesses, armies etc.) it does not count. You can get this trophy after becoming emperor by just letting your game run. Having armies around is expensive so a tip is to "fire" your armies (while in town returning them to free workers) if you don't need them anymore to save a lot of money. Once you have 50,000,000 then the trophy will pop, this is also a good moment to go for Military State.