In order to grow the rare Gold Crops in Rune Factory 2, you must have access to the Seasonal Chambers on the 5th Floor of the Palermo Shrine and you must have the Inquisitive Waltz Spell. The Inquisitive Waltz really is the KEY to many items in Rune Factory 2, from the Broken Hilt to a Gold Pumpkin.
To use the Inquisitive Waltz, you must clear the chamber of ALL Monsters, and destroy the Generator(s). You then equip the Spell and activate it. The Tame Monster accompanying you will search for hidden items.
I made a video about this, which shows the process. I added it to another post, but here is the link again:
The video shows the Trieste Forest but the process is the same everywhere.
On the 5th Floor of the Palermo Shrine, you will find a Chamber for each of the Four Seasons that contains a Garden. Here you can grow Crops. Indeed you MUST grow Crops in order to unlock the final mystery of the Shrine...
Apart from this, however, each of these Chambers contains a hidden item point that can be discovered using the Inquisitive Waltz.
Here is a new video specifically about the Gold Pumpkin:
As always, when the Inquisitive Waltz is used, you only have a chance of finding a rare item. Most of the time, you will obtain ordinary items... The rare items in the Seasonal Gardens are the elusive Gold Crops. Use your Seed Maker to convert a Gold Crop to seed and then plant the seeds to grow Gold Crops yourself!