The upcoming Christmas events should help you with gems. They usually have a 23 hour refresh repeatable that gives gems in addition to the ones from the other maps.
I only use gems for equipment that will be near the best in my camp & for playing the Christmas 2015 & 4th Birthday. I felt pretty short on gems in the early regions, but China was better.
I usually only by a pack of it has two good equipment items (2 slots each). A couple of times I've bought ones with one equipment item & event material to get a second (but that only improves 3 slots).
If willing to spend a little but not much, the equipment pack when you enter a region usually has really good equipment.