If you hover over the trade route icon at the top of the screen before and after taking the policy, you'll see it increase immediately. But if you look at your overall GPT, or hover over the GPT and look at gold "from trade routes between your cities and other civilization's cities", that doesn't change until the next turn.
The same is true if you buy a caravansary, or manipulate the value of a trade route by switching the source city to gold focus. It seems the game only recalculates your overall trade route income at the start of each turn.