ᐅ Trap installed too low, drain pipe has no slope. What can be done?

Dear community,

We have been experiencing an issue for some time now where the drain under the kitchen sink gurgles. The water drains normally, but the gurgling is new.
We have checked the trap for clogs—everything is fine.

We then noticed that the drain pipe connected to the trap has no slope.
In the photo, you can see the current setup. We built a cardboard structure under the trap to slightly raise the drain pipe. Unfortunately, this did not help.

Do you have any ideas on how to fix this problem?
We installed a new sink about 3 years ago. We assume the basin is deeper than the previous one, which likely means the required slope of the drain pipe is no longer sufficient. We didn’t notice this issue during the installation back then.
We are also puzzled that the gurgling only appeared after 3 years.

I appreciate any tips or help.

Thank you very much & good luck.