Forme-Change Items
To access the GP items, you must visit the Gold Shop on the third floor of the Shop's directory.
There are two types of items that you may buy at the shop using Gold Poké: GP Vouchers, and Forme-Change Items.
GP vouchers are used for turning your GP into a redeemable item, so you can give them to someone else for redemption. Forme-Change items, on the other hand, cause certain pokemon to change their Forme when given as a held item. However, one of these items is necessary for the evolution of a certain pokemon.
The Vouchers come in values of 100, 550, 1,250 and 3,000.
The Forme-Change Items include the Weather Jewels, the Sparkplugs, and two others.