Gaddock Teeg
Legendary Creature — Kithkin Advisor
Noncreature spells with mana value 4 or greater can't be cast.
Noncreature spells with 
in their mana costs can't be cast.
Greg Staples
Ultimate Box Topper (PUMA)
• #U21Rulings
A spell's mana value is determined only by its mana cost. Alternative costs (including casting a spell "without paying its mana cost"), additional costs, and cost reductions don't affect a spell's mana value.
(wotc • 2018-12-07)
If one half of a split card has a mana value of 3 or less and doesn't have an {X} in its mana cost, Gaddock Teeg lets you cast that half.
(wotc • 2007-10-01)
Rulings
A spell's mana value is determined only by its mana cost. Alternative costs (including casting a spell "without paying its mana cost"), additional costs, and cost reductions don't affect a spell's mana value.
(wotc • 2018-12-07)
If one half of a split card has a mana value of 3 or less and doesn't have an {X} in its mana cost, Gaddock Teeg lets you cast that half.
(wotc • 2007-10-01)