How can I save tax on Long term capital gain on stocks ? I have made a huge gain this year by inadvertently selling all my stocks in a panicky situation.

Well unless you have some losers to sell to offset the winners you are in a pickle.  If you can reduce your AGI by making a deductible IRA contribution then that may help.  

I agree with @Critter-3; there is not much you can do at this point.

If you feel comfortable with more unconventional types of investments, you could look into QOFs (opportunity zone funds) if you are within the requisite time period for investing in one.

See https://www.irs.gov/credits-deductions/businesses/invest-in-a-qualified-opportunity-fund