Gala won the first-leg of their round 16 tie 1-0 thanks to Mario Lemina's goal.
Slot said afterwards, "I think we started the game really well, even better than three months ago. We were having three or four great moments, with, of course, the biggest moment when Florian (Wirtz), I didn't see it back but it came across as almost an open goal where he could finish that ball.
"We weren't able to score. Like last time, when we had our chances they five seconds later got their first attack, led to a corner.
"You have to give credit for the way when they get a chance they play as if it is the last chance of their lives. That is definitely something we can learn from them because we sometimes get our chances and it comes across as if we think we get 10 more.
"So we were 1-0 down and it's already a difficult place to go if you're 0-0, but if you are one goal down it makes it even harder for us, for everyone who's on the pitch, except for their own players. Yeah, a 1-0 loss."
On failing to take their chances, Slot also said: "I wouldn't say frustration but that's a situation we are facing and I'm responsible for that. I'm facing already large parts of this season with getting the ball to the final third, getting them towards moments where you think you can get more.
"But today we've created enough to score a goal. And, of course, we scored one but that one was overturned by the VAR."
Slot, however, is confident of progress knowing they host Gala in the second-leg.
He said, "What I do know is now it's half-time. We've lost here twice with 1-0 and the good thing is the next game is not played here. It is played at Anfield and our fans can create a similar atmosphere.
"I don't think it's possible that if I look at the last two games that so many things can go against us as happened in the last two games, although sometimes I question that if I look at this season. But I think things will be more normal in a home game for us. And I also think we can improve our performance and that, with the help of our fans, has to lead to a big game in a week."