Any enters-the-battlefield abilities of the copied creature card will trigger when the token enters the battlefield. Any "as [this creature card] enters the battlefield" or "[this creature card] enters the battlefield with" abilities of the chosen creature card will also work.
Although the token is attacking, it was never declared as an attacking creature (for the purposes of abilities that trigger whenever a creature attacks, for example).
If the copied creature card has {X} in its mana cost, X is considered to be 0.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Slawomir Maniak
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
The amount of mana you spent to cast Gyrus is usually equal to its mana value, normally 3 plus the value chosen for X. However, you also include any additional costs you pay, including the commander tax.
Gyrus's token copies exactly what was printed on the original creature card and nothing else. It doesn't copy any effects that modified it before it was put into your graveyard.
You choose which player or planeswalker the token is attacking. It doesn't have to be attacking the same player or planeswalker that Gyrus is attacking.
If another creature enters the battlefield as a copy of Gyrus, consider how much mana was spent to cast that creature to determine how many +1/+1 counters it enters with.
If Gyrus enters the battlefield without being cast, then no mana was spent to cast it. It enters the battlefield without any +1/+1 counters. If no other effects are increasing its toughness at that time, it will be put into its owner's graveyard as a state-based action.
