Commercial CA certificate for GitLab version 12.2.4-ee

Currently, we are using GitLab Enterprise Edition version 12.2.4-ee without subscription on CentOS 6.8 OS. We are planning to buy Commercial third-party Trusted CA certificate to avoid “TLS/SSL certificate problem”

Could you please provide any recommendation & cost of CA certificate purchase which is compatible with GitLab?

I can recommend Let’s Encrypt. In most cases it’s easy to get a cert from them, they cost absolutely nothing, and I believe there’s documentation out there for setting it up in a variety of situations.