Turn to Frog overwrites all previous effects that set the creature’s base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after Turn to Frog resolves will overwrite this effect.

The creature will lose all other colors and creature types, but it will retain any other card types (such as artifact) or supertypes (such as legendary) it may have.

Effects that modify a creature’s power and/or toughness, such as the effect of Titanic Growth, will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness and effects that switch its power and toughness.

If the affected creature gains an ability after Turn to Frog resolves, it will keep that ability.

If one of the Theros block Gods is affected by Turn to Frog, it will be a legendary 1/1 blue Frog enchantment creature with no abilities. If it stops being a creature, perhaps because your devotion to its color(s) decreased, it will be a legendary blue enchantment with no abilities. The way continuous effects work, the God’s type-changing ability is applied before the effect that removes that ability is applied.

Turn to Frog doesn’t counter abilities that have already triggered or been activated. In particular, there is no way to cast this spell to stop a creature’s ability that says “At the beginning of your upkeep,” “When this creature enters the battlefield,” or similar from triggering.