GC stands for Guidance Counselor. Guidance Counselor is your teacher who is supposed to guide you through the application process. Just like an applicant, the guidance counselor will also have to create an account in Common Application where he/she will submit your letter of recommendation, provide info about the school and you, if necessary upload a document stating your financial difficulty to pay application fees among other things.
Using common app is free. However, you may have to pay certain fee to university/college when you submit an application. The fee depends upon the university. But that fee can be waived if you ask your GC to submit an statement saying that paying the application pay is a financial burden to your family. Most of the Nepali students ask their GC for this statement. There is a section in common app where your GC can upload it.