I am big gyre fan, but do not know the gyres from this brand, but prices seem to be attractive. Looks like they have three models, 60 gallon, 135 gallon, 265 gallon.
Given that gyres lose strength over time until fully cleaned, I would recommend going with the middle model even though you only have 50 gallon. The other argument would be that the cheaper models flow speed are sometimes overstated, and running a more powerful device at lower speed is probably better than a lower powered device at a higher speed.
Only risk in my opinion is that these devices power settings are not well adjustable, meaning the lowest power settings could still be too strong.