A person who supports gold, and its financial products such as gold certificates and gold ETFs, as an investment and hence is particularly across the board bullish on the commodity gold as a venue of investment and or a measure of wealth. A goldbug might also be a person who advocates a return to the gold standard or any other form of currency regime in which the value of gold is the standard.
In specific contexts, the term bears pejorative connotations as it is used to refer to someone who is obsessed with the idea that there is a conspiracy relating to gold and silver to the effect that influential governments manipulate their prices and stockpiles to their advantage.
