Your config file is missing TLS and SSL settings. You would need to set this up and confirm your gateway is working on HTTPS before proceeding with mTLS. You can use a keyless API as a test.
After confirmation, proceed to configuring your API definition using Static mTLS. I see you have already enabled keyless and mutual tls in your API definition. Everything looks fine however you are missing an entry for the client_certificates. You can find out how to add the ID of the client_certificates here.
Considering you have an org_id, the final certificate ID would be a concatination of your org_id + the generated SHA256 fingerprint. For example basicc3a4b867915e56b23f4f945112dd22d15eda22715135ba8f7aa28734e1a8c514. If no org_id is specified in the API definition then the final client_certificate is c3a4b867915e56b23f4f945112dd22d15eda22715135ba8f7aa28734e1a8c514.
Hope this helps