About Ishvara matter of the Bhagavad-gita.......

Īśvara :-

The main philosophical subject matter of the Bhagavad-gita is the
explanation of five basic concepts or truths:

*** Ishvara (The Supreme Controller)-
Īśvara , Ishvara or Eashwara (Sanskrit : "the Supreme Ruler; the Personal God") — is
Brahman associated with Maya but has it under His control unlike the jiva who is
Maya's slave. He has a lovely form, auspicious attributes and infinite power to create,
sustain and destroy. He dwells in the heart of every being, controlling it from within. He
responds positively to true devotion and sincere prayer. When God is thought of as the
supreme all-powerful person (rather than as the infinite principle called Brahman), he is
called Īśvara or Bhagavān .

Description :-

Most Hindus, in their daily devotional practices, worship some form of this personal
aspect of God, although they believe in the more abstract concept of Brahman as well.
Depending on which aspect of Īśvara one is talking about, a different name will be used
—and frequently a different image or picture. For instance, when God is spoken of as the
creator, God is called Brahmā. When spoken of as preserver of the world, God is called
Vishnu. When spoken of as destroyer of the world, God is called Shiva.
Many of these individual aspects of God also have other names and images. All the
various deities and images one finds in Hinduism are considered manifestations of the
same God, called Īśvara in the personal aspect and Brahman when referred to as an
abstract concept. In their personal religious practices, Hindus worship primarily one or
another of these deities, known as their "ishta devatā," or chosen ideal. The particular
form of God worshipped as one's chosen ideal is a matter of individual preference.
Although Hindus may worship deities other than their chosen ideal from time to time as
well, depending on the occasion and their personal inclinations, they are not required
to worship—or even know about—every form of God. Hindus generally choose one
concept of God (e.g., Krishna, Rama, Shiva, or Kali) and cultivate devotion to that
chosen form, while at the same time respecting the chosen ideals of other people. devis
The Hindu scriptures speak of many individual deities, called devas. Ishvara is just the
name used to refer to the personal God in general, when no particular deity is being
referred to.
The devas (also called devatās) are an integral part of the colorful Hindu culture. These
various forms of God are depicted in innumerable paintings, statues, murals, and
scriptural stories that can be found in temples, homes, businesses, and other places.
The elephant-headed deva known as Ganesha is worshipped before commencing any
undertaking, as he represents God's aspect as the remover of obstacles.
The most ancient Vedic devas included Indra, Agni , Soma , Varuna , Mitra , Savitri , Rudra,
Prajapati, Vishnu , Aryaman and the Ashvins . Vishnu and Shiva are not regarded as
ordinary devas but as Mahādevas ("Great Gods" ) because of their central positions in
worship and mythology.
Some Hindus consider the various deities not as forms of the one Ishwara, but as
independently existing entities, and may thus be properly considered polytheistic.