If you are familiar with other TCG, Gold Foil cards could be considered akin to holographic cards. Goil foil cards are nearly identical to their standard foil counterparts, with a couple additional benefits. They share the same stats, abilities, and mana cost in battle, their benefits all apply outside the battlefield.
These benefits, combined with the scarcity of supply, make these cards tend to be worth their digital weight in gold.
Gold Foil Cards enter the game through the same means as any other card, only at a much more scarce rate. Whenever a card is revealed, whether from a pack or reward chest, it was a 2% chance to turn into gold.
The chance of a gold turning gold can be increased through the use of an Alchemy potion. These potions may be bought from the shop or randomly dropped in reward chests.
Because gold foil cards are so much rarer than standard foil, they are worth notably more collection power. Gold foil Cards are worth either 25x or 50x their standard bcx. All cards from the Untamed set and older receive the 50x bonus, while newer reward cards and sets starting with Chaos Legion are 25x power.
When you use gold foil cards in ranked battles, a little bit of their golden nature rubs off on your earnings. You will receive a 10% DEC bonus for each gold foil card used in battle, this enables up to 70% DEC bonus just from using gold cards. It's possible to get any extremely large bonus if the right combination of gold foils and Alpha/promo cards, well over 100% extra DEC.
In addition to their other benefits, Gold Foil cards start at a higher level and require less BCX in order to level up. It takes substantially less gold foils to achieve max level, although it may not be the most cost effective.
While Gold Foils do start at higher levels, they still scale to your summoner level. If you are using a level 1 rare summoner, your Gold Foils will be level 1 as well.
Gold Foil cards are Splinterlands own touch on holographic cards, with some additional benefits that back up their value and scarcity. In my opinion they are a nice flair to the game that add a nostalgic excitement to opening packs and reward chests. Always putting you on the edge of your seat for the potential any reveal holds.