Gigantomancer

• WWizards of the Coast2010-06-15
  Official Ruling
  This ability overwrites all previous effects that set the targeted creature’s power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after this ability resolves will overwrite this effect.
• WWizards of the Coast2010-06-15
  Official Ruling
  If a leveler is affected by Gigantomancer’s ability, then enough level counters are put on it to reach a level indicated by a new level symbol, it will still be 7/7, not the power and toughness indicated by that level symbol.
• WWizards of the Coast2010-06-15
  Official Ruling
  Effects that modify the targeted creature’s power or toughness, such as the effects of Giant Growth or Glorious Anthem, will apply to it no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness (such as +1/+1 counters) and effects that switch its power and toughness.

Rulings via Scryfall