That size is ok, provided that your GCC is out at least a few hours a day and has a chance for exercise, and an out of cage area to play in (more than just a cage top).
Recommended cage sizes are constantly going up (when I got my GCC 18 years ago, all the cage recommendations were for 16x16 to 18x18"). IMO, quality time outside of the cage is much more important than cage size, at least once the cage is big enough for a bird to have some toys and still have room to stretch its wings (which the cage you are looking at does have).
But if you aren't sure about being able to give enough out of cage time consistently (for example if you travel a lot and your bird must stay caged while you are away), then you really should try getting another size larger cage. You won't regret getting a larger cage.