Hello Soumya Singh Verma

‘I was gone’ emphasises the state of having gone and that ‘I had gone' emphasises the act of going.  In 'I was gone', 'gone' has the role of an adjective.

'I was gone' is grammatically incorrect. In English, you can say either 'I was gone' or 'I was home' but not 'I was gone home'. This is because the verb was followed by either home or gone represents the place where the speaker is. That is, the speaker was somewhere. To say was gone home incorrectly uses was.

The second sentence is in correct simple past perfect form, and is grammatical.

Hope it helps you.