Where can we buy merits with just gold?

Merit Medals can only be bought with real life money. However, if you accumulate a lot of gold, I recommend you save them all for "Lucky Treasure" Events. This is when you can spend gold to spin a wheel.

The best way to spend gold in this game is to either

1) Buy Advanced Ports to attack others and move around the battlefield

2) Lucky Treasure (It's gambling, but it's the ONLY SOURCE of merit medals for free to play players). You also get level 4/5 officer cases and level 4/5 dummies along with the S officer fragments and cosmetics.

So tl;dr use your gold on LUCKY TREASURE WHEEL - It is the best and only source for free to play players. Merit Medals are by far the best items in the game because officer ranks ultimately dictate army strength.