What is the best thing to spend gold on?

It's up to you. But I rather spend them on VIP level. You get nice buff from it.

Can't go wrong with advanced/deep teleports, aegis, and march acceleration. If you aren't fighting, you are playing Farmville. If you aren't protecting your troops, they are the Farmville vegetables. Deep battle is the most fun part of the game. Go kills stuff!

If you are starting out and trying to build everything up, you might not be ready to attach other players. Instead, you will want to make sure that you are "Bubbled", as they say, with a shield (called "Aegis" in the game) - especially during the Silver event (Thursday 7PM EST through Sunday 7PM EST). To do that, you want to make sure you have enough 'Aegis'. You can buy them with Gold - also with Silver when the event is over (Sunday night) - and also with Corps points in the Corps store. Apart from needing to have an Aegis - everything else that is useful can be obtained in due time from quests and events.

An old question, but if you still play, here is my recommendation:

Spend gold on:

1) Advanced ports and 8 hour aegis bubbles (These should only be bought from the arms dealer who will give 20-60% discounts on items you find in the item store) - These 2 items are your bread and butter when hitting other players

2) Lucky Treasure - It's an event where you spin a wheel to obtain random rewards. It's a stupid gamble at first sight, but it is the ONLY effective method to obtain "Merit Medals" which will rank up your officers. When attacking, what will really tilt the battle in your favor assuming you're fighting enemies with the same troop tiers as you will be your officers.

Higher officer ranks (upgraded via merit medals) means stronger buffs.

Good luck!