Amnesty International said yesterday that the death sentences were handed down Monday, along with life sentences for three other soldiers.
The rights group called the death sentences a "cruel violation" and said they should be commuted to prison terms. It also called for a new trial.
Lt Malick Sanyang, a spokesman for the Gambian Armed Forces, declined to comment when asked about the sentences yesterday.
The Dec 30 power grab was put down by forces loyal to President Yahya Jammeh, who himself came to power in a coup in 1994.