I have used the CLI and the quotes worked fine for me. Here is the complete command I have used:

gcloud beta compute backend-services update app --custom-request-header 'X-Client-Geo-Location:{client_region},{client_region_subdivision},{client_city}' --custom-request-header 'X-Client-Geo-Region:{client_region_subdivision}' --custom-request-header 'X-Client-Geo-LatLong:{client_city_lat_long}' --custom-request-header 'X-Client-TLS-Version:{tls_version}' --custom-request-header 'X-Client-TLS-Chiper:{tls_cipher_suite}' --custom-request-header 'X-Client-Hostname:{tls_sni_hostname}' --custom-request-header 'X-Client-RTT:{client_rtt_msec}'