Thanks for the help guys, I've just submitted a help request to both Coinbase and Gemini, so hopefully they get back to me soon so I can get this sorted out. If I recall correctly, the total amount that was traded couldn't have been more than $300 total, as I only had 0.01 ETH last I checked in my offline wallet. It is entirely possible that I did trade at a loss, but I just want to be certain that I don't actually owe these people anything.
We can't know for sure whether you had any tax liability without knowing the exact timeline. Did you only buy in 2017, and then start selling in 2018? If so, you probably had no taxable net gains for the year. Even so, assuming you were required to file a tax return for 2018, you technically were required to report these transactions on Form 8949 and the net gain/loss on Schedule D.
The good news is that these amounts are so low that you are unlikely to be on the IRS' radar, and your activity is unlikely to have thrown off your tax liability by much, if at all. I would still recommend recovering your trading history and running the numbers, if only for peace of mind. In this situation, it's probably not worth poking the bear and amending for such a low level of activity.