In the CW drama's seventh season premiere, the creatures took control of Castiel (Misha Collins).
"We'll develop what the Leviathans mean this season," Gamble told AOL TV. "Like the other bad guys who emerge a little as time goes on, their personalities will become clear in the next few episodes."
The executive producer added that the unseen monsters - also known as 'the Old Ones' - are "having fun" and are "here to have a good time".
"They're not... here to hail fire and brimstone on the world and just be moustache-twirling evil," she said. "[That's] something... that we wanted to definitely weave in with these characters."
Gamble also suggested that Castiel may not be dead, despite the Leviathans' claims.
"You should never take anyone for their word that anyone is dead, especially when they're trying to get you out of their way," she hinted. "You certainly should not just ring the final bell on seeing Cas or Misha just because you don't know what episode he'll show up in next."
Supernatural continues on Fridays at 9/8c on The CW.
> Supernatural exec: 'Sam might lose his marbles'
Watch a clip from the Supernatural premiere below: