An airdrop is an income transaction and if this token is listed on our price sources (Fiat, coin and commodities price sources, individual exchange price, change Fiat account Currency) then it will have a cost base of course at the time you receive it. If you do not want to see it on the gains page please use the filter option "half checked" on top:
You can exclude income transactions on the tax report as well as described here if allowed in your country: Exclude Staking and other income rewards on tax reports