To harvest a gold pumpkin you have to acquire the fertilizer 2 in the tech tree and be able to craft it. once you have fertilizer 2 in your inventory go and buy 4 silver pumpkin seeds from the farmer in South wheat field ( he does not have gold pumpkin seeds). Now plant the fertilizer 2 first and you will see a sign with a gold star, plant your silver pumpkin seeds on that same square and let it grow. Now just harvest the fully grown pumpkin and the achievement should pop right away. I haven't seen anyone sell gold pumpkin seeds but if you do that would work as well.
Equipment needed= hand mixer, alchemy work bench 2, distillation cube 1, church workbench
Recipe= 2 flavor enhancers and 10 peat.
Flavor enhancer= ash + toxic solution + life extract in that order.
Ash= body cremation
Toxic solution= red mushroom or orange jelly
Life extract= maggot,yellow flower or heart
Peat= crop waste at compost in garden
You must study all ingredients first before mixing.
Note: fertilizer 2 will not turn a bronze seed into gold it will make it silver.