Grand Company missions are indeed a quick way of leveling crafters and gatherers. Later on the relevant beast tribes and custom deliveries are also good.

But the very simplest way is to simply craft one of each item. (You get some nice bonus XP the first time you craft an item). And of course gather yourself what is needed for this crafting.
You can then use, sell, or desynth the crafted items.
Only a small handful of ingredients that you will need to buy this way, and you will craft all the gear you need along the way. And what you don't need yourself you can sell for a nice profit.