This was despite the fact they picked up a crucial 1-0 win over Hamburg.
The club's first victory of 2014 lifted them out of the Bundesliga's bottom three and above their rivals from the north, but the manner in which they took three points was not to Gentner's liking.
"It's incredible - indescribable," he told Sky. "We've played a few good games lately, but today's was perhaps our worst performance in an eternity.
"We were totally insecure and you could feel the fear for the entire game.
"Despite everything, though, we worked hard for our win and that is all that matters."
Stuttgart could slip back into the bottom three on Sunday, while they are only one point above Hamburg with a lot of work still to be done to secure their top-flight status for next season.
"We now have eight more finals and we've got to treat them as such," coach Huub Stevens said.
"Every game is a final for us. These three points help us, of course, but we've still got a long way to go."
According to Gentner, though, the three points only help to a certain extent.
"It's not as if we've really won back any self-confidence with the way we've played," he said. "Maybe we were scared of winning because things have gone wrong so often lately, but we've come through that."