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.
Quicksilver Gargantuan
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.
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Steven Belledin
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
