Genju of the Fields — ruling 1

Activating the ability that turns it into a creature while it's already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.

Genju of the Fields — ruling 2

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Genju of the Fields — ruling 3

When activated multiple times, it will have the triggered ability multiple times. When the animated Plains deals damage, the ability will trigger once for each copy of the ability. If, for example, you activate the ability three times, the Plains will deal two damage, and you'll gain six life.

Genju of the Fields : règles, fonctionnement et interactions (MTG)

Genju of the Fields 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

Enchant Plains : Until end of turn, enchanted Plains becomes a 2/5 white Spirit creature with "Whenever this creature deals damage, its controller gains that much life." It's still a land. When enchanted Plains is put into a graveyard, you may return this card from your graveyard to your hand.

How it works : Genju of the Fields

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
Activating the ability that turns it into a creature while it's already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.
No other card is named in this ruling.
#2August 1, 2008· Wizards of the Coast
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
No other card is named in this ruling.
#3August 1, 2005· Wizards of the Coast
When activated multiple times, it will have the triggered ability multiple times. When the animated Plains deals damage, the ability will trigger once for each copy of the ability. If, for example, you activate the ability three times, the Plains will deal two damage, and you'll gain six life.
No other card is named in this ruling.