1. Save money
2. Invest money
3. re-invest profits from investments
4. buy GTR once annual profits from investments are larger than annual loan repayments, insurance, rego, maintanence, modding, etc.
Unfortunately this may take a while, and some sacrafice....the good thing is the bank will be happy to loan you money based on the value of your investments, and after you've paid off the GTR loan you still have your investments with which will make a nice deposit for a house...