Can I confirm that this is the ROOT certificate? It says PUBLIC Certificate in the text but the heading says "trusted root certificate".
Yes, a root certificate is required to configure Application Gateway's HTTP Settings.
Assuming this is the CA/Root Certificate, I have tried all tools to convert it to CER and upload it and it still says "Certificate Data Invalid". I have CER file which is basically a text file with ---begin--- and ---end--- . Is that the right format? or CER expected should be in a binary format?
The .cer file should be in base-64 encoded format. If needed you can follow the instructions in document shared above to create a custom Root CA certificate using OpenSSL.
Hope this helps. Please let me know if you have any additional concerns. Thank you!