Gold is the most universal helper in the game.
It can be used to buy things like Energy for Kingpins, Items or speed-up investments and constructions.
The most effective way to use gold is:
To buy family items.
You will need a lot of family items (Liquor, Handcuffs, Shackles), they come rarely. Speedups and such things you will get a lot of.
So best idea to use gold is to buy the family items for leveling up your family buildings. They are important to level up, because they are the key to level 25 investment-center. And in the end this is the key to unlock T4.
When possible buy those familiy items for 10k because it is the best exchange rate.
Start leveling up with your faction hall. The faction hall increase your raid capacity and it is depending the level of the hotel.
After you have your first family building on level 25 the others will come faster. At least it feels like this 
.
By the way - most stupid idea to use gold: Train troops.
Troops can get killed when you are under attack. When you spend gold for troops you dont loose only troops with getting attacked, you loose that gold too. Not a good idea!