When will the doctor put me on insulin or other medication for gestational diabetes?

Depends. It depends on your current degree of control. If you are running very high, Insulin will be needed. If slightly above goal, a pill will be fine. If currently controlled and at goal, diet/exercise will be all you need but you have to be followed closely as your numbers will go up as the pregnancy progresses.
Diet first. The first step in managing gestational diabetes is diet and walking after meals. If that doesn't work, your doctor may need to add Insulin or a pill like glyburide. Usually those are started if more than 1/2 your sugars are above target.
Keep sugar normal. To keep your glucose normal and reduce the risk of damage to the fetus.