Typically exports are used to gather USD. It sounds like Ghana has gold. Presumably people want their gold? They could sell the gold for USD, then use that to purchase the oil.

But using the gold for currency does sound like it could be more efficient, cutting out the middleman.

Note: normally countries maitain a float of foreign reserves for this, and I'm guessing this stash of economic lubrication has run a business dry for Ghana and they don't have the luxury of rebuilding their reserves, and hoping to use gold as a more immediate solution.

Implication? Only a handful of currencies are accepted in international trade.