Just pray they don't give you a 1099 with grossly incorrect values like they generously did for me.

Was it a 1099-K? [1] It reports gross payments, not profits. The IRS knows this; it's their form.

[1] https://www.irs.gov/businesses/understanding-your-1099-k

I know that this is the total transactions and not profits, and the numbers are way, way off. I have a local copy of all my trades, of course, and they match perfectly with the transaction history available on GDAX. But the numbers that Coinbase reported on this 1099-K are completely wrong. They literally put the wrong numbers in the 1099-K. And as one would expect, dealing with Coinbase support is about as much fun as being set on fire. I fear that Coinbase is going to end up costing me a lot of time and money because of their botched accounting.

They'll start doing this more and more. Just be careful though because the moment you have a single transaction outside of Coinbase, their reports will be incorrect. You may want to consider checking out cross-platform tools like https://www.cointracker.io which help you keep track of all your cost basis across exchanges and wallets automatically.