Gita Mahathmyam 
By Adhi Sankara
Translated by P. R. Ramachander
[The Gita Mahathmya which normally one reads after reading Gita is from Varaha Purana. I also found this prayer in the web. It is not listed in the Complete Works of Adhi Sankara. You can see another translation of this prayer at: www.jagannathavallabha.com/gita/html/gita_mahatmya.html. The summary meaning of one more version, which gives Mahathmya chapter wise is given at: omshivam.wordpress.com/sreemad-bhagvad-gita/gita-mahatmya-the-glories-of-the-bhagavad-gita-from-the-padma-purana/]
1. Gita Sasthram idham punyam,
Ya padeth prayatha Pumaan,
Vishno padam avapnothi
Bhaya Sokadi Vrajeth
That gentleman who reads with humility,
The science of Gita which is blessed,
Would reach the feet of Lord Vishnu,
And would be bereft of sorrow and fear
2. Gita adhyayaa sheelasya,
Pranayama paraasya cha,
Naïve Santhi hi paapani,
Poorva janma kruthani cha.
He who makes reading of Gita a Habit,
And also practices the Pranayama (breathing exercise)
Is not affected by sins, even those of previous births.
3. Mala nirmochanam pumsaam,
Jala snanan dhine dhine,
Sakud Gitam bhasi snanam,
Samsara mala nasanam.
People have to take bath,
In the water day after day to remove dirt,
But by taking bath once in the waters of Gita,
He can get rid of the dirt accumulated by the domestic life.
4. Gita, sugita karthavya,
Kim anyai Sastra vistaraihi,
Yaa swayam Padmanabhasya,
Mukha padmad vinisrutha.
It is our duty to sing the Gita,
Which originated from the mouth,
Of Padmanabha lying on a lotus flower,
And there is no need for any other Sastra in detail.
5. Bharath Amrutha Sarvasvam,
Vishnor vakthraad vinisrutham,
Gita Gangodhakam peethwaa,
Punar janma na vidhyathe.
Gita which is the entire nectar of Mahabharatha,
Is the one which fell out in the words of Lord Vishnu himself,
And so if to drink Gita which is like the Sacred waters of Ganga,
You would not have one more birth.
6. Sarvo upanishadho Gavo,
Dogdha Gopala Nandana,
Partho vatSah Sudhir bhokthax,
Dugdham Gita amrutham mahad.
All the Upanishads are cows,
The one who is to milk them is the boy Gopala,
Arjuna is the calf and those who have pure intellects are consumers,
And the milk is the great nectar of Bhagwad Gita.
7. Yekam Sastram, Devaki puthra Gitam,
Yeko devaa Devaki puthra yeva,
Yeko mantra thasya namani yaani,
Karmapi yekam thasya devasya seva.
The only religious book is Gita by the son of Devaki,
The only one God is the son of Devaki herself,
The only chant is the name of him, himself,
And the only job is to service to that God.