Hello Michael,
I faced the same issue in my Gate.io API sync regarding a deposit of 500USDT on 2021-12-23. I am not aware if the fault lies on Gate.io API or in Koinly API, but you can safely soft delete it (or permanently delete it if you do not aim to sync Gate.io from beggining ever again) since that transaction is an airdrop of USDTEST tokens and not an actual USDT token. It is a zero value test token and you have no taxation obligations towards that.
You can review it from your side by logging into your Gate.io account, open the “Billing Details” menu item in website’s sidebar, and searching for your USDTEST airdrop transactions. There could be as well negative airdrop transactions that deduct the same amount airdroped before (e.g., in my case I can see a -500USDTEST on 2021-12-31 but for some reason there are not displayed in Koinly).
PS: Regarding Gate.io API sync, it is also worth mentioning about a token mismatch issue that leads to wrong calculations as well. The GatePoint (POINT) token which is used internally in the platform for the deduction of fees is incorectly matched to the Point Network (POINT) token…