Gold bars are pivotal to the story line and important to your achievements. You’ll be introduced to the gold bars at the start of the second level. Each level after the first has a total of 5 gold bars that need to be found, you can check your progress on them by hitting start and then checking “Gold Left On This Map.”
Every gold bar is accompanied by a “Acta Non Verba” sign nearby. You’ll be introduced to the sign at the start of the second level and from then on you should keep and eye out for the sign. They’re easy to spot as they are white and red and most things around are either green, blue, or brown.
I recommend collecting all the gold on your second play through of the game so that you know the level better than your first play through.
For a great guide on finding all the gold check this guide out this guide.