Gold is the main 'currency' of the game. Gold is gained through trades and taxes paid by the population. It is one of the most important elements in the game. Military upkeep and researching Technologies require gold.
If the total military upkeep cost is more that the gold production (gold/hr), the gold production will go negative. This situation can be countered by disbanding some of your military units.
In order to increase gold production you need more population for which you may need to construct or upgrade buildings.
Some research topics also boost gold gain; several tasks give gold as reward.
Buildings such as Tax Office boost the gold production. The 'Stock Exchange' wonder also boosts tax income by 20%.