Can I get into the chemical engineering program at IET Lucknow with a general of 20K and a GNGL of 2,500?

Honestly, the answer is No. One of my friends was in a similar situation and he did not get admission with a rank closer to this.

With the GNGL rank of 2,500, it would be very difficult for you to get into the chemical engineering program at IET Lucknow. The general rank is way higher.

I would strongly suggest you not settle for any random private college with this rank. It would be better to take a year drop.

However, if you have other options such as JEE Mains, you can try private colleges like LNMIIT, Thapar, etc.