Your Radiant Flames was cast with only black mana, so it will only deal 1 damage to each creature.
CR609.4 explains how abilities that say you can do something “as though” some condition were true work–for the purposes that specific effect lays out you treat the game as though that condition were true, but for all other purposes, the game is treated normally.
Gonti, Lord of Luxury says “you may spend mana as though it were mana of any type” to cast
Radiant Flames, so the mana is being treated as mana of any type for the purpose of spending it, and as 609.4 lays out,
only for the purpose of spending it. For all other purposes–such as the purpose of checking what color that mana was in order to determine how much damage
Radiant Flames should deal–the game is treated normally. The game therefore sees that all the mana you spent was black, and
Radiant Flames only deals 1 damage to each creature.