I love the Bhagavad Gita's take on everything and I am very pro-Krishnaism, however I am slightly confused on the Gita's view on other religions. Does the Gita claim that the "god" described in Christianity, Islam, etc. is ultimately the same in the form of the Supreme Godhead? Is this interpretation correct?
@Krishna's comment above to your question is right; the Gita was spoken long before any of the other great world religions were around.
Krishna says (Gita IV. 7-8):
Whenever there is a decline of dharma, O Bharata, and a rise of adharma, I incarnate Myself.
For the protection of the good, for the destruction of the wicked, and for the establishment of dharma, I am born in every age.
So Krishna says that he will incarnate Himself again and again when needed; many people believe that Krishna Himself incarnated as Christ. To answer your question, yes, the same God worshiped in other religions is the same Supreme Godhead of the Gita.
and in Gita IV. 11:
In whatsoever way men approach Me, even so do I reward them; for it is My path, O Partha, that men follow in all things.
God does not belong to one religion, all religions belong to God. You can approach Him through all paths, all religions lead to Him. Different approaches will have different views of the same thing, one view does not invalidate the other. Ramakrishna Paramahamsa said (Gospel of Sri Ramakrishna):
He who is called Krishna is also called Siva, and bears the name of the Primal Energy, Jesus, and Allah as well--the same Rama with a thousand names. A lake has several ghats. At one the Hindus take water in pitchers and call it 'jal'; at another the Mussalmans take water in leather bags and call 'pani'. At a third the Christians call it 'water'. Can we imagine that it is not 'jal' but only 'pani' or 'water'? How ridiculous! The substance is One under different names, and everyone is seeking the same substance; only climate, temperament, and name create differences. Let each man follow his own path. If he sincerely and ardently wishes to know God, peace be unto him! He will surely realize Him.
And Gita (VII. 7):
There exists nothing whatsoever higher than I am, O Dhanajaya. All is strung on Me as a row of gems on a thread.
All the different religions are simply gems strung around the Supreme Godhead.
First addressing your confusion about Gita's view on other religions.
Gita has never opinionated on any religion; not even Hinduism! Historically the term "Hindu" itself was originated in Iran. Ofcourse, Hinduism carried the Gita forward. (Don't mistake "Hinduism" to be a religion, it's a "way of life", according to Indian Supreme Court.)
The only (falsely interpreted) mention about other "Religions" is here:
sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja ahaṁ tvāṁ sarva-pāpebhyo mokṣayiṣyāmi mā śucaḥ
Abandon all varieties of religions and just surrender unto Me. I shall deliver you from all sinful reactions. Do not fear.
Since there is no single word translation in western language for Dharma, people often label "Religion" as "Dharma" which not right, though Religion plays important part in defining Dharma. Dharma can be appx termed as "Conformance of own duties with righteousness".
Now answering your question:
NO Gita does Not claim that god described in Christianity and Islam are ultimately same in form of Supreme Godhead.
For below reasons:
- Islam & Christianity did not exist when Gita was said. Though some scholars like Zakir Naik has claimed that Islam existed with beginning of humanity, there is no evidence of that.
- Gita advocates that "Formful God" is easier to achieve than "Formless God". It directly contradicts with Islam. You may refer beginning of Chapter-12
- With respect to Christianity, many concepts are matching with Hinduism like Oneness of god who created everything, trinity, multiple gods. But again, there is no such claim in Gita.