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.
7 Dec 2018
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.