In what ways could imperialism be considered an economic phenomenon?
The phrase, "god, gold and glory" dates back (at least) to earliest days of the Age of Exploration, in the early to mid 1400s, when Spanish and Portuguese sailors, backed by the largesse of their monarchs, set out on increasingly far flung journeys in search of trading routes and mythical cities filled with gold. As the phrase above suggests, these early European explorers, like all explorers before them, were motivated by the desire to spread their faith (in some cases), to bring glory to their country, but most of all, to find riches, whether those riches were actual gold, land, or raw materials.
Columbus convinced Queen Isabella and King Ferdinand to fund his voyages based largely on the premise that he could find a faster route to India, which would allow the Spanish to gain a larger piece of the Spice Trade, which was already lucrative, but required long, treacherous overland journeys through often unfriendly territories. Specifically, the Spanish hoped to procure more indigo, pepper, cinnamon and silk. Once Columbus ended up in the New World, however, and not in the East, as he had promised, he was able to justify continued voyages financed by the Spanish crown by bringing back gold and slaves.
Later, Conquistadors like Hernan Cortez went to the New World with the explicit purpose of securing land and finding out whether that land could be farmed or otherwise exploited for material gain (gold and jewels). The Spanish Encomienda system was created to maximize the amount of territory that relatively few Spaniards could claim, and to send a percentage of those earnings back to the mainland. Meanwhile, the Portuguese established Brazil as a giant plantation.
In the 1500s and 1600s, when the English East India Company and Dutch East Company overtook their Iberian predecessors as the main explorers/imperialists of Europe, they did so for purely commercial reasons, as the names of those Joint Stock Companies imply. The entire Mercantile System, of course, was predicated on having a positive balance of trade, and that meant selling more raw materials to one's trading partners than one bought.
We know that imperialism was about projecting power and increasing territory. Yet tellingly, no country ever fought over Greenland, the Arctic, or the Antarctic (until recently, when it became clear that oil and shipping lanes were up for grabs). Nor were there fights over the Gobi Desert or the Sahara. In other words, no country ever built an empire for "god or glory" if gold was not also available.