To grow a gold-star Pumpkin you will need 4 gold-star Pumpkin seed (or 24, if you want to use a Zombie farm).
You can't buy a gold-star Pumpkin seed from the Farmer, so the only way to get it is:
1) Buy 4 silver-star Pumpkin seeds.
2) Use a Quality fertilizer II on an empty garden bed (increases the chance of getting a gold-star seed).
3) Plant 4 silver-star Pumpkin seeds.
4) Harvest the crop.
5) Repeat 1-4 until you get enough gold-star seeds.
Of course, planting the seeds on several gardens beds at once speeds up the process.