How do I get Gold?

Ask owner of the site

Why can't I finish some buildings with Gold?

Due to game play reasons the buildings Residence and Palace are excluded from this function. The same applies to villages in which the wonders of the world can be constructed.

I activated the Plus account, however, my resource production did not increase.

The production did not increase because Plus is separated from these additional features.

Do I need to activate Plus before I can use any other function?

No, you can use every single gold feature as long as you have enough gold.