SplinterGlossary: Gold Foil
Gold foil cards (often referred to as GF) are much more scarce than their regular counterparts. Without modifiers, chances are that 2% of the cards found in regular editions turn to gold foil. Using the alchemy potion, that chance doubles. For loot chests, this chance depends on the type of chest. More details here.
Gold foil cards offer a 5% bonus on winning matches in ranked battles and much fewer are needed to upgrade to higher levels. A 1-BCX gold foil card also starts from a higher level than its regular counterpart (depending on the rarity).