After replacing the battery and upon connecting horn is sounding?

It sounds like you have a security system that is engaged. I would do a hard system reboot; this guide will show you how:

https://www.2carpros.com/articles/step-by-step-guide-performing-a-hard-system-reboot-for-your-vehicles-electronics-systems

This guide can help as well:

https://www.2carpros.com/articles/how-to-reset-a-security-system

Please go over these guides and get back to us.

Is the battery good, does it have 12 volts? Also let's check all fuses with the system on.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

This guide can help:

https://www.2carpros.com/articles/car-battery-load-test

You may have had a voltage surge and taking out the GEM. we should do a CAN scan to confirm. You can get a CAN scanner (Controller Area Network) which will work on most cars from Amazon.

Here is a video to show you how:

https://youtu.be/u-4syLc-ifQ

and

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Here is the location of the GEM so you can check it out and replace it if a code comes up for it, you can get a preprogrammed unit by searching Google or Ebay. Check out the images (below). Please let us know what happens.

They are working for us, can you check again?