psalms
On the website Britannica, Psalms are described as "book of the old testament composed of sacred songs, or sacred poems meant to be sung. In the Hebrew Bible, psalms begin the third and last section of biblical canon,..."
The reference here is to show one of the things monks do in the world, despite the apparently low opinion of them as presented in the story.
phlegmatic
Phlegmatic is defined by Cambridge dictionary as "a phlegmatic person does not usually get emotional or excited about things." The word of which is used to convey how calm Gargantua is, even as a small child.
barbican
On Merriam-Webster, a barbican is defined as "an outer defensive work especially a tower at a gate or bridge" so they're in reference to the wall and/or tower that guards the castle before him.
"Barbican." Merriam-Webster. https://www.merriam-webster.com/dictionary/barbican
joust
Jousting is something that Arthur and his knights are shown doing quite frequently when they weren't on their many quests. A video and transcript on Britannica.com, Nic Maher notes what the sport entails, "jousting is when two knights, fully decked out in very heavy armor, charge at each other with big sticks called lances. And they do it all while trying to hit each other as hard a possible."
"Learn about the medieval sport of jousting." Encyclopaedia Britannica, Encyclopaedia Britannica, Inc https://www.britannica.com/video/193338/sport-jousting#:~:text=Jousting%20is%20when%20two%20knights,other%20as%20hard%20as%20possible.
Trouble and ignorance are gone! the Light Hath come unto me, by Thy favour, Lord! Now am I fixed! my doubt is fled away! According to Thy word, so will I do!
After their long discussion about the ethics and spirituality of Arjuna's duty to battle his family and friends, the warrior decides to fight. As the article "What should be the Moral Responsibilities of the Individual: A philosophical Discussion on the Ground of the Bhagavad Gita" notes about Arjuna's duty in the end "...Krishna reminds Arjuna that because he (Arjuna) belongs to the warrior class, it is his duty to fight." Within the dame passage also notes that his duty to fight as he ultimately decides here, is something he must do and it is just.
http://www.katwacollegejournal.com/issues/Prakash%20M-Philosophy-What%20should%20be.pdf
What does it tell us not just about the religion, but the culture from which the religion emerged?
The Gita overall has and important place in the Indian culture as it throughout states a lot of the core tenets of Hindu belief. An article about the work on Britannica notes some of the these beliefs ranging from the immortality of the soul, to devotion to taking action devoid of desires and the holiness of doing so. (https://www.britannica.com/topic/Bhagavadgita)