A powerhead provides a constant line of direct flow. Some powerheads have ball joints which mean that they can be swivelled into whatever direction you need. Not all powerheads swivel, such as Ecotech Vortech, so you may have to play around with it’s placement.
A gyre, also known as a wavemaker, provides a broad, linear stream of flow in one direction. This then bounces off the other side of the tank, creating a flow in the opposite direction. The entire column of water can be displaced in this way which, depending on your rockwork, may eliminate the need of a powerhead.
FYI: The term ‘gyre’ means large rotating current, as found in the ocean.