Click the network selection drop down at the top of the MetaMask extension and select "Custom RPC".
Fill in all the fields like below:
Leave the rest blank.
This is the same as adding ETH to your MetaMask wallet, but send GO instead. You can purchase GO in many place, check here for a list.
You might see a zero value, but this is because the cost is so small and MetaMask doesn't show that many decimals. Just press Confirm anyways and it should work.