Students at the Senior School are offered a wide and varied academic curriculum so that they are well placed to reach their full potential. They are prepared for the ACT Year 10 High School Certificate and the ACT Year 12 Certificate, under the auspices of the Department of Education and Board of Senior Secondary Studies respectively.  CGGS is also an International Baccalaureate (IB) World School authorised to offer the IB Diploma Programme for students in Year 11 and 12 .

Students in Year 7 study a broad range of subjects. The core subjects of English, geography, history, ITC, mathematics, physical education/health, religion and philosophy and science are undertaken for the full year.   Drama, music, textiles and design, and visual art and design are undertaken for a semester each, and in addition, students choose two languages from French, Japanese, Mandarin and Latin to study for a semester each. 

As students progress through to Year 12, they are able to make more choices about the subjects they elect, although English, mathematics, science, physical education/health and religion and philosophy remain core subjects through to the end of Year 10.

The ACT Senior Secondary system caters for students in Years 11 and 12, generally aged between 16 and 19 years of age. It is based on the principle of continuous assessment throughout Years 11 and 12 and governed by the policies and procedures of the Board of Senior Secondary Studies (BSSS).

A student qualifies for the award of a n ACT Senior School Secondary Certificate on completion of an educational program approved by the School. Students who wish to go to university must complete a tertiary package and sit the ACT Scaling Test (AST), to receive an Australian Tertiary Admission Rank (ATAR). An accredited package is for students interested in direct entry to the workforce from school and/or studying at TAFE.

Students generally study five subjects in both years. English is compulsory in Years 11 and 12, while mathematics is compulsory in Year 11 only.

Students who complete a tertiary package and who qualify for an ATAR, receive a Tertiary Entrance Statement in addition to their ACT Senior Secondary Certificate. It contains information which may be used in applying to tertiary institutions. A requirement of completing a tertiary package is sitting for the AST in Year 12.