If the chosen creature has a characteristic-defining ability that sets its power and/or toughness, that ability will not be copied. For example, if Quicksilver Gargantuan copies a Nightmare, it will not gain the ability that sets its power and toughness to the number of Swamps you control. As such, it will remain a 7/7.

If the chosen creature has {X} in its mana cost (such as Protean Hydra), X is considered to be zero.

If the chosen creature is copying something else (for example, if the chosen creature is a Clone), then your Quicksilver Gargantuan enters as whatever the chosen creature copied, except for its power and toughness.

If the chosen creature is a token, your Quicksilver Gargantuan copies the original characteristics of that token as stated by the effect that put the token onto the battlefield, except for its power and toughness. Your Quicksilver Gargantuan is not a token.

Except for its power and toughness, Quicksilver Gargantuan copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

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

If Quicksilver Gargantuan somehow enters at the same time as another creature (due to Mass Polymorph or Liliana Vess's third ability, for example), Quicksilver Gargantuan can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

You can choose not to copy anything. In that case, Quicksilver Gargantuan simply enters as a 7/7 creature.

If Quicksilver Gargantuan is not a creature (for example, if it entered as a copy of an animated land), it will not have the characteristics of power or toughness at all, so it won't be 7/7. If it later becomes a creature, its power and toughness will be determined by the effect that causes it to become a creature; again, it won't be 7/7.