Gigantoplasm

The activated ability that Gigantoplasm gives itself doesn't have a duration. It lasts until Gigantoplasm leaves the battlefield or another effect overwrites it (for example, if you activate the ability again once you have more mana available). Each time the ability resolves, it will overwrite any previous effects that set its power and toughness, including previous activations of the ability. Effects that otherwise modify Gigantoplasm's power and/or toughness, including +1/+1 counters, will still apply no matter when they took effect.

If the chosen creature is a token, Gigantoplasm copies the original characteristics of that token as stated by the effect that created the token. Gigantoplasm isn't a token, even if it's copying one.

The activated ability overwrites all previous effects that set Gigantoplasm's base power and/or toughness to specific values. Other effects that set its base power and/or toughness that start to apply after the ability resolves (including ones created by subsequent activations of the ability) will overwrite the effect of the ability.

However, the effect of that ability isn't copiable. That is, if Gigantoplasm is a copy of a creature with base power and toughness 2/2 and you activate its ability making it a 4/4 creature, another creature that becomes a copy of Gigantoplasm will have base power and toughness 2/2.

If Gigantoplasm somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

Any enters-the-battlefield abilities of the copied creature will trigger when Gigantoplasm enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the chosen creature will also work.