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)

Relevant cards

Note: All applicable cards may not be listed below.
BW2_72
DP7_6
HGSS3_3