How to farm in GD?
Is leaving the game the only way to reset the areas?
You kill stuff > Get loot > Profit ?!?!
And yes, that is the only way.
Also the crucible is pretty effective/efficient once you have a character that can farm it. (Ignore this if you don’t have the dlc)
Unfortunately yes.
Note that (from what i’ve read here), there’s a mechanism in place to prevent (or limit) the same item (epic/legendary or blueprint) from dropping several times in the same game session (which means until you quit and reload) so if you plan to farm several areas and you’re interested in those drops (rather than just components or MIs) it’s worth doing them all in a single session.
Most efficient way is to farm Crucible repeatedly in the same session - you get the biggest amount of Legendary items per minute and via the method of elimination/reduction of duplicates you are guaranteed to get every possible item in the game if you lets say do 80+ runs of crucible in the same session.
In the past i even put my PC on sleep mode just to keep the same Cruci session and farmed like 3-4 hours per day and would have gotten every item if the game didnt crash at one point 
its old school like in d2.
You think about a farm “route”. open a new game and go berserk on your route.
Right now for me is something like this:
Warden
Cronley
Undead Dungeon
Bastion of chaos
Ancient Groves
Reset or end the game. What i like.