What is Karma-Yoga of Bhagavadgita?

Karma yoga is learning(Gyana) by doing(Karma). This experiential learning is more if we work detached to the fruits of karma and remaining conscious that we are working with a spirit of service as per our pure nature.Service melts our ego and we get merged in the universal experience of God.

It is interesting to elaborate on the meaning of "Yoga" before discussing further about Karma Yoga.
Few lines from wikipedia regarding "Yoga" are as under:

"1) Yoga (Sanskrit, Pāli: yoga) is a physical, mental, and spiritual discipline, originating in ancient India. The goal of yoga, or of the person practicing yoga, is the attainment of a state of perfect spiritual insight and tranquility while meditating on the Hindu concept of divinity or Brahman. The word is associated with meditative practices in Hinduism, Jainism, and Buddhism."

"2) Within Hindu philosophy, the word yoga is used to refer to one of the six orthodox (āstika) schools of Hindu philosophy."

"3) Yoga in this sense is based on the Yoga Sutras of Patanjali, and is also known as Rāja Yoga to distinguish it from later schools. Patanjali's system is discussed and elaborated upon in many classical Hindu texts, and has also been influential in Buddhism and Jainism. The Bhagavad Gita introduces distinctions such as Jnana Yoga ("yoga based on knowledge") vs. Karma Yoga ("yoga based on action")."

"4) Other systems of philosophy introduced in Hinduism during the medieval period are bhakti yoga, and hatha yoga. The Sanskrit word yoga has the literal meaning of "yoke", from a root yuj meaning to join, to unite, or to attach. As a term for a system of abstract meditation or mental abstraction it was introduced by Patañjali in the 2nd century BC. Someone who practices yoga or follows the yoga philosophy with a high level of commitment is called a yogi or yogini."

In view of the above explanations about the "yoga", the karma yoga of Gita is a way, a method, a state (of body & mind) and also a destination that we must seek to achieve our FINAL DESTINATION, "UNION" with God.

KS- Yoga means joining together .What can we join in our actions/karma- knowledge/Gyana!
Yoga is not exercise- it is subtle.

Karma means Work or karya and yaga means Exercise .
For taking mastery in work Or your duties do exercise thats make you perfect in your field
That is Karma yoga in Bhagavadgita.

Doing 'niyata'(allotted task) karma with "Gyana"(with knowledge of God & Universe) - leaving the results to God - doing it as a prayer to God - invoking Him and requesting Him to receive and accept the result of the KARMA (our work) - since this is "Yagna of Karma" - sacrifice of work and its result to God - so that neither the work nor its result (good or bad) will attach themselves to our atma (self) in this birth or hereafter - so that in future, we will have less or no 'sanchita' (accumulated) karma - so that one day it becomes zero of sanchita karma, which possibly clear one of the major hurdles to achieve Moksha. Am I correct?

Karma with Gyana(of oneness of universe) is Yoga of Gyana in Karma(Gyana yukta acharan),

The main problem is whatever you say about the ultimate truth is only a part of the truth. What you say about Karma Yoga is correct and it does not exhaust all those sayings which can be said about Karma Yoga. But, that should not deter you from saying some correct thing.