The Forgotten City

Probably near the middle of the Forum area, if close enough will start an automatic dialogue (forgot guy’s name. Starts with D) and if you did Fabia’s quest before him, he’ll take notice of your wooden bow and suggest partnering into getting riches with said bow. He’ll paint it gold and there will be a shrine to goddess Diana also holding a gold bow and he’ll ask you to go switch bows. He’ll open the doors to the shrine and of course you might guess where this is going.
Long story short this probably one of the earliest places to get the achievement. Simply escape and along the way there will be a “whisper” telling you to shoot the algae and that will gild the area. Another note is things you’ll be able to gild will be hovered over by bright glowing bugs.