If I received a CP200 showing underreported income due to Coinbase's 1099-k & submitted all 30K worth of transactions with 10k as taxable gain. What do I send to fix it?

If you did have the gain suggested by the IRS just pay the calculated tax and return the signed form. 

Please find out if you purchased Audit Defense for the CP 2000 tax year.

If so, please call TurboTax for  Audit Defense contact information. 

If you do not have Audit Defense for the said year, it is  recommended that you amend the tax return, adding the capital gain transactions to check the amended result if agree with IRS calculation. The additional capital gain might impact your state return, if applicable. TurboTax Premier is the correct version to handle capital gain(loss) transactions.

Please complete the notice response form before the due date.

For more details, please read the IRS article "Understanding Your CP2000 Notice" below, including the FAQs.

https://www.irs.gov/individuals/understanding-your-cp2000-notice

TurboTax Audit Support Center
https://support.turbotax.intuit.com/irs-notice/audit-support/

I got a CP2000 notice from the IRS

https://ttlc.intuit.com/community/irs/help/i-got-a-cp2000-notice-from-the-irs/00/27061

do you believe the 10K isn't taxable? explain your reasoning.

No audit defense available.

I submitted all the transactions as recorded by Coinbase, Through those transactions, I had a Capital gain of 10k which is what I also reported in TurboTax. 

I have seen where Coinbase is no longer sending out 1099-k's because of the IRS's confusion in understanding what is reported.

The 10k is taxable but the IRS form is saying that the 30k reported on the 1099-k from Coinbase is taxable. I submitted all the transactions as reported by Coinbase. After all gains and losses are tallied there is a 10k profit, which is what I entered in TurboTax.