Attacks » Gyro Ball
What happens if I use Bronzong's "Gyro Ball" attack but I don't have any benched Pokemon?
You have to switch Bronzong if you are able to; the "if you do" part refers to when you have no Benched Pokemon. So if Bronzong doesn't switch the opponent's Active Pokemon doesn't switch either.
Source: Rebel Clash FAQ (2020-04-18), TPCi Rules Team (2020-04-30)
Attacks » Gyro Ball
The version of "Gyro Ball" attack on Forretress says "you may switch..." as an optional action, but the version of "Gyro Ball" on Ferrothorn indicates the switching part of the attack is mandatory. Which way is correct?
The effect may vary from card to card, so you should play each attack as it is written on its card. So for Forretress the switch is optional, but for Ferrothorn it is mandatory.
Source: Emerging Powers FAQ (2011-09-01), PUI Rules Team (2011-09-01)