In addition to the 'two cages' comment, I would add this:
GCC's are a fair bit more intelligent than budgies (parakeet). This means they need more stimulation to be kept properly happy, which in turn means they need a somewhat larger cage to accommodate the extra toys etc. If they don't have enough to keep them busy and interested, they can get frustrated and grumpy.
You don't
have to spend a fortune on toys (though you surely can lol) - many homemade toys work great. One of my favorite foraging toys for smaller birds, is to cut off about a 4 inch section of a paper towel roll, fold in one end, stuff it with shredded paper and treats (dry treats, not fresh/wet foods, obv), fold in the other end, and tie the package shut with a few strands of cotton yarn. The first time or two, I usually stick some millet sprigs or something in the yarn to get them interested in it, but once they get into it and figure out there's food hidden inside, they will love destroying them.