Are you sure? I thought the "chemical imbalance" story was made up by pharmaceutical companies to justify their new class of antidepressants.
https://qz.com/1162154/30-years-after-prozac-arrived-we-stil...
https://qz.com/1162154/30-years-after-prozac-arrived-we-stil...
What GP gets right, though, is that depression != sadness. It is normal (and often healthy) to feel sadness after experiencing loss or other hardships. It is not healthy to feel hopeless and devoid of energy for months on end... that's what depression is.