His room would be sprawled with fabrics, maybe reuse past costumes and clothes hanging through his room. "The stuff outside is too expensive for the quality" that's why he makes them. To make it in time, it's why he starts from summer...
I like to think that Halloween is a big thing for Gaku's siblings. He would gather all of them in the summer, ask them what costumes all of them like and "Would Try to make it happen". It's all on a list for October.