You need to request a code from Brisbane Centre prior to the reporting point at the Gold Coast. On the Sprinbrook sector frequency (this will be in ERSA and your VTC, from memory 119.5) all you do is call up Brisbane Centre and say "Brisbane Center, Alpha Bravo Charlie Cessna 172 at Jacobs Well, 1500 to Gold Coast". I always tack on request code, but that part is not necessary because they have your flight plan and know your intentions. Brisbane centre will reply with "Squawk 1234 remain outside controlled airspace" and you will read back exactly that, then enter the code into your transponder.
Once you have set your code and once Brisbane Centre has identified you on the radar, they will call you "Alpha Bravo Charlie, Identified 2 mile south east of Jacobs Well report altitude" and you will respond "1500 Alpha Bravo Charlie". Then Brisbane Centre will respond with the instructions about contacting Gold Coast Tower at Q1. At Q1, its the same process as Class D.
What Aldo says is what I would do now, but for training purposes its best to keep it simple. I also thing the Class G at Southport is valuable experience as a student, especially with an instructor next to you. Also, getting clearance into Brisbane airspace is rare for GA. i have been refused more times that I have been cleared!
NOTE THIS. In Class D and C, the tower provides separation. Class C, separation is provided with SSR, Class D, separation is procedural (ie they dont have you on a radar screen). As a VFR aircraft, separation is ultimately your responsibility because you can fly anywhere you want, ATC only provide you with traffic information to assist in separation. Positive separation is only provided to IFR aircraft, because they fly set routes in the sky.