You are correct, when you purchase a CD through your brokerage and hold it to maturity, there will be no gain.
However, in your case, your cost basis is not the face value of the CD, it is the FMV of the CD on the date of death. The market value of CD's held by a broker (unlike traditional bank CD's) fluctuate primarily due to changes in market interest rates. So the value of the CD was more when you inherited it than when it matured, this is why there is a gain reported.
For example if the CD that you inherited had a 5% interest rate but the market interest rate was 4.5% on the date of death, the CD would be worth more than face value at that time to account for it's higher rate.
As the CD gets closer to maturity, it's value will always gradually move to it's face value and be worth exactly it's face value when it matures.
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