Battlegate Mimic — ruling 1

The effect from the ability overwrites other effects that set power and/or toughness if and only if those effects existed before the ability resolved. It will not overwrite effects that modify power or toughness (whether from a static ability, counters, or a resolved spell or ability), nor will it overwrite effects that set power and toughness which come into existence after the ability resolves. Effects that switch the creature’s power and toughness are always applied after any other power or toughness changing effects, including this one, regardless of the order in which they are created.

Battlegate Mimic — ruling 2

The ability triggers whenever you cast a spell that’s both of its listed colors. It doesn’t matter whether that spell also happens to be any other colors.

Battlegate Mimic — ruling 3

If you cast a spell that’s the two appropriate colors for the second time in a turn, the ability triggers again. The Mimic will once again become the power and toughness stated in its ability, which could overwrite power- and toughness-setting effects that have been applied to it in the meantime.

Battlegate Mimic — ruling 4

Any other abilities the Mimic may have gained are not affected.

Battlegate Mimic : règles, fonctionnement et interactions (MTG)

Battlegate Mimic is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever you cast a spell that's both red and white, this creature has base power and toughness 4/2 until end of turn and gains first strike until end of turn.

How it works : Battlegate Mimic

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
The effect from the ability overwrites other effects that set power and/or toughness if and only if those effects existed before the ability resolved. It will not overwrite effects that modify power or toughness (whether from a static ability, counters, or a resolved spell or ability), nor will it overwrite effects that set power and toughness which come into existence after the ability resolves. Effects that switch the creature’s power and toughness are always applied after any other power or toughness changing effects, including this one, regardless of the order in which they are created.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
The ability triggers whenever you cast a spell that’s both of its listed colors. It doesn’t matter whether that spell also happens to be any other colors.
No other card is named in this ruling.
#3August 1, 2008· Wizards of the Coast
If you cast a spell that’s the two appropriate colors for the second time in a turn, the ability triggers again. The Mimic will once again become the power and toughness stated in its ability, which could overwrite power- and toughness-setting effects that have been applied to it in the meantime.
No other card is named in this ruling.
#4August 1, 2008· Wizards of the Coast
Any other abilities the Mimic may have gained are not affected.
No other card is named in this ruling.