The gyre has a single shaft that runs from one side of the pump to the other side. This means that it is not possible for only one side of the motor to work. What this also means that the issue is that propeller or the bushings on the propeller are either defective/stripped or most likely, not installed correctly.
The propeller has two bushings installed that allow the propeller to properly grip the motor's shaft. The first is a rubber boot that goes into the end of the propeller. The second bushing is a small black cap that gets inserted into the rubber boot.
The motor shaft is then inserted into the small black cap. One must be careful to make sure that the flat side of the shaft (yes, the shaft has a flat side) is properly lined up with the flat side marking of the small black cap bushing.
Most likely, one or both of the points above are the issue. Please ensure that all the bushings are installed and that the shaft is properly aligned with the propeller bushings. If not, the shaft cannot grip the propeller correctly and only one side of the gyre will work.