The most authentic commentaries on Bhagavad Gita both in Hindi and English and even Sanskrit are available with Gita press, Gorakhpur, India.
A commendable commentary by Bhagavad Gita from authentic writers like Jayadayal Goyandka, Swami Ramsukhdas and Hanuman Prasad Poddar are available at a very subsidized prices ranging from $1-$3. Gita press is fond of selling books at a price lower than manufacturing cost.
Commentaries on Bhagavad Gita and 11 principal Upanishads by Adi Shankaracharya (an enlightened one) is available in Sanskrit and also English.
If we seriously desire indulging in Bhagavad Gita, we must rely on commentaries available with Gita press only. The original gist of Bhagavad Gita containing original 700 shlokas verses in Sanskrit is available for mere $.10. What more can we expect from Gita press?
My personal advice is instead of browsing through the online store of Gita press; if possible one should personally go to a Gita press store available in major towns of India and look for self what suits our palate and thinking style. Gita press has many varied editions.
I am not related to Gita press in any way yet I have found their commentaries to be most authentic. Many commentaries are available in market written by people who did not know ABCD of spirituality, can such authors or the translations themselves can be considered authentic?
Established people in different walks of life particularly in India had a knack for writing a commentary on Bhagavad Gita or a mere translation. This was only to satisfy their false ego. These poor people failed to realize that path of spirituality can be tread demolishing ego to zero.