Your initial assumption is not correct.
To get Gold you will need to get Silver first.
So:
- You start at 0
- When you reach 2 A3 flights and 12000 miles OR 24 000 miles without A3 flight within 12 months, you will be Silver
- From that day you need 4 A3 flights and 24 000 miles or 48 000 miles without A3 flight within 12 months
So if you start at 01 September 2018, you reach Silver 15 March 2018 and then reach Gold 30 September 2018. You will be Gold from 1st Oct 2018 to 30 September 2019.
You will need to get your 4 A3 flights + 12 000 miles or 24 000 miles without A3 flight during that period. As soon as you reach it the status is extended to the next year, starting 1st October 2019.