Really speaking, the Gita teaches us everything. You don't need to learn anything else besides the Gita. If you understand the Gita properly, then the only additional thing you can do is meditate on it --  not merely study the words and memorize the slokas.

Memorizing is good, but unfortunately people think that memorizing the slokas is everything. Oftentimes memorization creates pride in you because you think that you know more than others or you are better than them. This is foolishness. We should learn humility from the Gita. Certainly we want people to learn and memorize the Gita, but not out of pride or for the feeling of superiority over others. The way to improve your life is to go beyond this and practice the Gita in your life. That is the only way you are going to have any benefits.

The Gita is a prescription. A prescription is not good until you buy the medicine and consume as recommended by the doctor. Similarly, the Gita is the prescription given to us for our diseases: ignorance, greed, lust, and pride. To get rid of these diseases you must not only understand the Gita, but practice it.

-- Swami Radhanandaji